How to tell when fish have outgrown a tank

[gcalberto]

I have a specific doubt that sparked this much more general question.

I have 6 clown loaches in a 45 gallon aquarium and I know they will eventually outgrown it. They are now at around 3-4 inches. What size should they get until this tank becomes too small for them? 

I also have an indonesian tiger fish (Datnioides microlepis) sitting at 2-3 inches, which is also bound to overgrow his current 45 gallon tank

What should I look for when deciding whether or not my fish have become too big for their tank?

[TheDukeAnumber1]

The first metric is water quality, when your filtration and maintenance schedule can no longer keep ammonia, nitrites, nitrates, ect in check something needs to be changed.

Second would be quality of life, if they're negatively impacted by the size of the aquarium, which is subjective and depends on setup so maybe someone else who knows more about those species can comment.

[gcalberto]

50 minutes ago, TheDukeAnumber1 said:

The first metric is water quality, when your filtration and maintenance schedule can no longer keep ammonia, nitrites, nitrates, ect in check something needs to be changed.

Second would be quality of life, if they're negatively impacted by the size of the aquarium, which is subjective and depends on setup so maybe someone else who knows more about those species can comment.

First of all, thanks for the answer!

Well, water quality is (at least for now) a non issue. the filtration I got should be enough for a tank twice that size, as long as it's not heavily stocked.

As for the other metric, there's nothing i have noticed for now, but I'd appreciate it if someone with more experience could tell me what exactly to look for when evaluating that

Edited August 3, 2020 by gcalberto

[TheDukeAnumber1]

15 minutes ago, gcalberto said:

First of all, thanks for the answer!

Well, water quality is (at least for now) a non issue. the filtration I got should be enough for a tank twice that size, as long as it's not heavily stocked.

Glad to help ( ͡ᵔ ͜ʖ ͡ᵔ ) One thing to note is that our filters don't know how big the tank is, it's all dependent on bio-load so be sure to make periodic water testing a habit. GL

[Brandy]

I would be watching for aggression, stress behavior like neurotic glass surfing or listless lack of movement, and pure awkward movement.

Like if you have a dead empty tank you would want the fish to to have plenty of space to turn around, so the tank should at least be 2-3x the length of the fish in all directions at a bare minimum. Also some fish move a lot, some less so, depending on what is normal for their species, so speed racers need room to run (swim). But then if you have a bunch of decor and there are tight spots that they get stuck in, it might be your first indication that they are getting "big" for their digs. 

Also, as @TheDukeAnumber1 points out, just because a filter is rated for a tank 2x the size doesn't mean it can remove nitrates from the water--if the fish start increasing bioload because they are eating/pooping more, then your water changes will have to come more often because there is still only x number of gallons to dilute that waste, and that is also a clue.

Edited to add:

All the fish I have seen that were obviously oversized for their tank were either doing the fish equivalent of pacing, or were lying around listlessly, playing statue. Hopefully you would have moved them long before it comes to that.

Edited August 3, 2020 by Brandy

[gcalberto]

11 minutes ago, Brandy said:

I would be watching for aggression, stress behavior like neurotic glass surfing or listless lack of movement, and pure awkward movement.

Like if you have a dead empty tank you would want the fish to to have plenty of space to turn around, so the tank should at least be 2-3x the length of the fish in all directions at a bare minimum. Also some fish move a lot, some less so, depending on what is normal for their species, so speed racers need room to run (swim). But then if you have a bunch of decor and there are tight spots that they get stuck in, it might be your first indication that they are getting "big" for their digs. 

Also, as @TheDukeAnumber1 points out, just because a filter is rated for a tank 2x the size doesn't mean it can remove nitrates from the water--if the fish start increasing bioload because they are eating/pooping more, then your water changes will have to come more often because there is still only x number of gallons to dilute that waste, and that is also a clue.

Thanks for the response!
 

I'll now know what to look for in the behavior my fish.

 

As for the filtration, I have a canister filter which holds a ton of bio rings and has a circulation of about 7x the volume of the tank per hour. I also have a powerhead connected to 2 largo sponge filters for extra circulation, biological filtration and oxygenation. This powerhead has a circulation of 4x the volume of the tank per hour. I feed my fish heavily and ammonia and nitrites are always 0. I used to measure nitrates before water changes, but even when I'd feed heavily for a week the nitrates would never go past 30-35ppm. I always do a 60% water change. 

I also have an air pump with an air stone for extra circulation, since I have some plecos that are very demanding regarding the amount of dissolved 02 in the water 

Edited August 3, 2020 by gcalberto

[Faedother]

Here is the ROUGH equation I use for a general idea. NOTE that the waste habit of the fish can change this. Also I use this for a single fish in a tank. If I have multiple fish, their aggression has to be taken into consideration. The more aggression, the less fish the tank can have.

Here are variables: Adult Length of the Fish (LF), Adult Width of the Fish (WF), Length of the Tank (LT), Width of the Tank (WT)

Length: 10*LF = LT

Width: 3*(LF+WF) = WT

Waste: This is usually set to 1 for most fish. However for fish like carp and koi, I set it to .5 or if the fish is a very messy eater I might set it to .25. For small fish, like ember tetras, I set it to 2 or 3.

Equation: (LT+WT)/Waste = Needed Tank size.

I know this is solution is math heavy and is not perfect because it does not account for surface area for oxygen exchange. But I hope it helps.

[Nataku]

Like Faedother, I usually use some math to help determine minimum tank size for a fish at a given size. Mind you, bigger is always better.

The minimum I use is the fish's length x 6 = length of tank.

The fish's length × 2 = width of tank.

So for example, a clown loach gets on average, 12 inches long. 12 x 6 = 72 inches ie a 6 foot long tank. 12 x 2 = 24 inches ie a 2 foot wide tank. Minimum, for an adult Clown loach.

But how does this help you know when to move up a tank size? I usually  add 1 or 2 inches to the current size of the fish for slower growers like clown loaches,  3 to 4 for faster growing species, and run the formula. So this formula is for the minimum size tank they should be in right now, because they are going to grow to this length within a year.

So let's say your clown loaches are 4 inches, I'd run the formula at 6 inches to accommodate for what they should be in right now.

6 x 6 = 36 inches and   6 × 2 = 12  inches

So a 3 foot long by 1 foot wide tank at minimum, right now.

But by next year? Add another two inches, so now the formula is 8 x 6 = 48 inches and 8 x 2 = 16 inches. Well I don't know of many 16 inch wide tanks, so bump that up to 18 inches and you're looking at a 75 gallon.

Continue the formula progression out until they reach adult size.

Also, things to look for on the fish itself that they are stressed and there's not enough room - look for damage on the end of the nose, top of the head. Scrapes, missing scales or swollen spots here, providing there is no other diseases to be worried about, are indications the fish is hitting their head on hard decorations, or on the end of the tank. Some fish will develop a neurotic tendency to swim from one end of the tank to the other, hit their head, turn around, swim to the other end, hit their head on that end, rinse and repeat. Its pacing, in fish.

Likewise, keep an eye out for tears or splits or worn edges on the pectoral and caudal fins, as these are likely to be the first ones damaged by them trying to squeeze into too small of spaces or of striking things when trying to turn around in too tight of a space.

 

[Daniel]

So when to a get a bigger tank?

These are the variables I use.

N = number of fish

T = number of tanks

D = Cost of larger tank

$ = Bank Balance

Equation = 2 ^N x T*cos(D*$)². If this number is not zero, and if a 29 gallon aquarium looks about the same to your spouse as a 40 breeder (substitute appropriately depending on spouse). Then it is time to get a larger tank.

[gcalberto]

1 hour ago, Nataku said:

Like Faedother, I usually use some math to help determine minimum tank size for a fish at a given size. Mind you, bigger is always better.

The minimum I use is the fish's length x 6 = length of tank.

The fish's length × 2 = width of tank.

So for example, a clown loach gets on average, 12 inches long. 12 x 6 = 72 inches ie a 6 foot long tank. 12 x 2 = 24 inches ie a 2 foot wide tank. Minimum, for an adult Clown loach.

But how does this help you know when to move up a tank size? I usually  add 1 or 2 inches to the current size of the fish for slower growers like clown loaches,  3 to 4 for faster growing species, and run the formula. So this formula is for the minimum size tank they should be in right now, because they are going to grow to this length within a year.

So let's say your clown loaches are 4 inches, I'd run the formula at 6 inches to accommodate for what they should be in right now.

6 x 6 = 36 inches and   6 × 2 = 12  inches

So a 3 foot long by 1 foot wide tank at minimum, right now.

But by next year? Add another two inches, so now the formula is 8 x 6 = 48 inches and 8 x 2 = 16 inches. Well I don't know of many 16 inch wide tanks, so bump that up to 18 inches and you're looking at a 75 gallon.

Continue the formula progression out until they reach adult size.

Also, things to look for on the fish itself that they are stressed and there's not enough room - look for damage on the end of the nose, top of the head. Scrapes, missing scales or swollen spots here, providing there is no other diseases to be worried about, are indications the fish is hitting their head on hard decorations, or on the end of the tank. Some fish will develop a neurotic tendency to swim from one end of the tank to the other, hit their head, turn around, swim to the other end, hit their head on that end, rinse and repeat. Its pacing, in fish.

Likewise, keep an eye out for tears or splits or worn edges on the pectoral and caudal fins, as these are likely to be the first ones damaged by them trying to squeeze into too small of spaces or of striking things when trying to turn around in too tight of a space.

 

Thank you for this explanation. It really helps a lot! 

My tank is 16 inches wide and 36 inches long 

Edited August 3, 2020 by gcalberto

[TheDukeAnumber1]

1 hour ago, Daniel said:

Equation = 2 ^N x T*cos(D*$)². If this number is not zero, and if a 29 gallon aquarium looks about the same to your spouse as a 40 breeder (substitute appropriately depending on spouse). Then it is time to get a larger tank.

It's a statistical certainly that that equation will not be zero lol.

[Bill Smith]

Bio-load is highly subjective and is dependent on clean water, not filtration levels.

3 hours ago, gcalberto said:

As for the filtration, I have a canister filter which holds a ton of bio rings and has a circulation of about 7x the volume of the tank per hour. I also have a powerhead connected to 2 largo sponge filters for extra circulation, biological filtration and oxygenation. This powerhead has a circulation of 4x the volume of the tank per hour. I feed my fish heavily and ammonia and nitrites are always 0.

This just tells us you have a filter which can make nitrates faster than most. IMO, the indicator of whether a bio-load is too much all comes down to the maintenance, not the filter.

Quote

I used to measure nitrates before water changes, but even when I'd feed heavily for a week the nitrates would never go past 30-35ppm. I always do a 60% water change. 

This tells us you are not overstocked. If you are changing your water to keep the nitrates down to acceptable levels at a frequency that you are willing to live with, you haven't exceeded bio limits. But as soon as you have to make more frequent water changes than you want to in order to keep the nitrates down, your bio-load has crossed the line. For some of us, two fish can be too much.

Adding plants reduces your need for water changes, and effectively buys you more bioload.

For myself, I am committed to 50% water changes weekly on all my tanks. I have automated enough of the water exchange process that I can live with the 2-3 hours of effort every Sunday. So I stock more heavily than most.

But five years ago, I had to struggle to motivate myself to change 25% monthly on one tank. And I was behind the rate of increased nitrates. So that tank was overstocked, even though it had only a handful of fish. 

Remember, it's about clean water, not about excess of equipment!

Bill

[gcalberto]

1 minute ago, Bill Smith said:

Bio-load is highly subjective and is dependent on clean water, not filtration levels.

This just tells us you have a filter which can make nitrates faster than most. IMO, the indicator of whether a bio-load is too much all comes down to the maintenance, not the filter.

This tells us you are not overstocked. If you are changing your water to keep the nitrates down to acceptable levels at a frequency that you are willing to live with, you haven't exceeded bio limits. But as soon as you have to make more frequent water changes than you want to in order to keep the nitrates down, your bio-load has crossed the line. For some of us, two fish can be too much.

Adding plants reduces your need for water changes, and effectively buys you more bioload.

For myself, I am committed to 50% water changes weekly on all my tanks. I have automated enough of the water exchange process that I can live with the 2-3 hours of effort every Sunday. So I stock more heavily than most.

But five years ago, I had to struggle to motivate myself to change 25% monthly on one tank. And I was behind the rate of increased nitrates. So that tank was overstocked, even though it had only a handful of fish. 

Remember, it's about clean water, not about excess of equipment!

Bill

Yeah, I forgot to say that, but I do weekly water changes. Hope that makes more sense 🙂