Issue with Cycle?

[Matt_]

Having a major issue in my planted 38 gal growout tank.  The water got cloudy, my Guppies and Plecos started swimming oddly (less active), Guppies flashing for a couple days, and today I had a mass loss of Bristlenose Plecos (lost over 15 today).  The Paleatus Cory adults and fry seem to be unaffected.  Nothing obviously wrong with the deceased plecos.  No issues in my other tanks also with the same types of fish using the same tap water for water changes.  The only thing that I can think of is that the cycle got out of whack or got reset (testing showed the cycle completed the first time), stocked too fast after cycling or overfed.  I have done a 50% water change for the last three days straight treating using Prime and added my pre-seeded Ehiem 2213 to the tank from another tank to try to add additional beneficial bacteria, but I am concerned that I cant water change out of the issue or it is not a cycle issue - I have resisted adding meds because I do not see anything yet.

Tank setup at the beginning of October and cycled using a pre-seeded Medium Sponge Filter from another tank, Ehiem 2213 with Small Coop Sponge Pre-filter, 2" sand bed (inert)

Constant Water Parameters:  7.4pH, 6 deg. GH, 4 deg. KH

Water Parameters: 11/15: 72F, 0.5 ppm Ammonia, 0.5 ppm Nitrite, 80-160 ppm Nitrate => Cloudy water, 50% Water Change, Turned up heater

Water Parameters 11/15: 76F, 0.25 ppm Ammonia, 0.5 ppm Nitrite, 40 ppm Nitrate => 50% Water Change, Added cycled Ehiem 2213

Water Parameters 11/16: 76F, 0.0 ppm Ammonia, 0.5 ppm Nitrite, 20 ppm Nitrate => 50% Water Change

Tap Water: 0.25 ppm Ammonia, 0.25 ppm Nitrite, 40 ppm Nitrate

Other Tanks: 0 ppm Ammonia, 0 ppm Nitrite, 20-40 ppm Nitrate

Fish Stocking level maintained for last 3.5 weeks:  10 adult Guppies plus fry, Bristlenose Plecos (30+ <1"), and 7 Paleatus Corys plus fry

I am looking for advice on what to do next.  I am also concerned adding the fish from this tank into my other aquariums in case the issue is also related to an illness.

[Cory]

Got a picture of the tank? it seems like it is a young tank with a high bioload. What I'm not understanding is, how did you have 40ppm nitrate in the water, change 50%, and end up at 20ppm if your tap water has 40ppm of nitrate in it?

[Matt_]

Good question, I had assumed the plants were exporting the Nitrates, but that probably does not add up here.  The tap water is from the Potomac and is probably also better represented as a range instead of a single value.  I rechecked the current values and I got Tap Water: 0.25 ppm Ammonia, 0 ppm Nitrite, 5 ppm Nitrate.  I was using the logged tap water values from a few days ago for the tap above.  The water parameters for the tank were taken before the 50% water changes.

I also forgot to mention that some the (all albino) bristlenose plecos also look washed out like they are in distress.  On the second photo the pleco on the left is showing normal colors and behavior and the pleco in the middle is washed out and is lethargic but still moving.

Edited November 17, 2020 by Matt_

[Hobbit]

That’s rough having so many nitrates in your tap water. ☹️ I think it’s petty clear that the main thing affecting your fish is nitrate poisoning. It’s possible they’ve developed secondary infections because of the stress, but if it were me I’d probably move some fish to my other tanks, as long as that didn’t overstock the other tanks and cause the same issue. Or just keep up the frequent water changes.

The good news is that your ammonia and nitrites are staying low. That means the beneficial bacteria in your cycle are still active and doing great! Your plants and denitrifying bacteria just need time to beef up their nitrogen-consuming capabilities. Algae will also step in to consume nitrates so you may be looking at an algae bloom soon if the nitrate level stays high.

Here’s an article about denitrifying bacteria if you’re interested: 

http://www.aquaworldaquarium.com/../../images/Discus.ico Aquaworld Aquarium - Article - Anaerobic Bacteria in Freshwater Aquariums - The Often Incorrectly Vilified Beneficial Bacteria

WWW.AQUAWORLDAQUARIUM.COM

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I usually stock my tanks pretty slowly (out of respect for my budget as well as tank stability 😅) because slow changes allow the system to adjust. I think if you keep testing and changing water, and don’t replace any fish right away, things will stabilize in time.

[Matt_]

Update, had continued loss of pleco grow outs, but I am still not totally sure what happened.  The cycle seems to have completed after letting the tank run without a water change for the last couple days (Nitrites are now at 0 again).  There are still a few plecos left in the tank, the corys did not stop breeding, and the guppies are still having fry.  All the guppies flashing and odd behavior turned out to be Ich today and I am treating with the Ich-X on hand, so I am hoping this tank will settle back down when the ich is gone.  Thank you for the help guys.