New fish in an old tank

[TomO]

I need to bounce a few questions off the group.
I have a 75 gallon planted Peacock/ Mbuna/ barb tank. I’m going to trade the cichlids for a few red gold severums . And a few electric blue acara an a couple geophagus . I already had 2 red parrots and 3 adult angels. The barbs are going to the 55 where the parrots and angels have outgrown. I realize the plants in the 75 will eventually be fodder for the severums. I’ll be removing the rocks and adding wood. I’ll also give the gravel a good vacuuming.

My question is, since there’s not a whole lot of beneficial bacteria in the water itself and catching 40 fish will likely stir up a bunch of inherent debris, can I just drain all 75 gallons? Then arrange wood in the new set up and add fresh water? Then transfer the angels and parrots to the newly set up 75? Or would the water need to be cycled first. I’ll be using the same sponge filters.

This is my current set up

 

[Widgets]

I would say that it is a definite maybe. I have brought a cycled sponge to a new bare tank QT setup using 50% old tank water and 50% new water, for an instant cycle.

You are planning on doing about the same, with the benefit of any beneficial bacteria that survives on the substrate and other tank decor. You will need to test the water often and be prepared to intervene if the cycle does not instantly resume. If the cycle does need to be restarted, I would expect it to go quickly since you have a decent head start.

[Guppysnail]

On 3/13/2022 at 7:59 PM, TomO said:

can I just drain all 75 gallons?

I do this for my Bn Pleco grow out and my guppy tanks every time I take them to the lfs.   I remove EVERYTHING so I can catch them. I also have done this several times to upgrade tank size. I don’t rinse out media so it takes everything with it. I feed lightly the first week or two in new larger tank.  The most I have ever seen is minor nitrite spikes. If I cannot take all existing filters I take at least some and monitor. The first week occasional traces of ammonia. Again I feed lightly until it settles. 

[Andy's Fish Den]

I would say that you are good with doing what you have outlined, as long as you don't plan on heavily stocking the tank right away. I have done it many times of draining all the water, and catching fish out, refilling and adding new fish and no issues. I would keep an eye on water parameters for the first week afterwards and do water changes if needed. 

[AndreaW]

At the end of the year I moved my 46G from upstairs to downstairs and to move it I had to drain all but about 1" of water but kept all the substrate, filter, media, etc. intact. (Fish were stored in a 5G bucket during the move). I added new water (probably comparable to a 90% water change. Those fish went back in the tank after the move, and I'm pleased to say everything survived and I maintained my nitrogen cycle (no ammonia or nitrite spikes).

Two weeks after the move, my sisters tank heater went out and she brought me all her survivors. (She had it in her garage and the water dropped to 55*F) Having nowhere else for them, they went in the 46G. I ended up overfeeding them, trying to nurse them back to health so I had some algae, but the cycle still stayed intact (still no ammonia or nitrite spikes).

With the same substrate and filter, I think you should be able to maintain your nitrogen cycle as long as you keep them wet. If you drain the water, fill a bucket to put your filter in (or anything else you are removing and plan to replace) to keep them submersed. 

You will likely lose some of your BB from removing the rocks and adding wood, but as long as you retain your BB in the gravel and filter, you should be off to a good start. Just keep an eye on your water for the first while and you should be pretty good.

Beautiful fish by the way!