Planted Tank Guru
Join Date: Jun 2004
Location: Baton Rouge, LA
It may not be as torturous for the remaining fish as you think.
I've seen some strange things in this hobby. One of the strangest was a 10G tank. 3G of it was filled with fish waste. No aeration or filtration. Never got cleaned. Never got water changes, only top-offs with unconditioned tapwater. The owner threw in a handful (!) of flake food twice a week. It was packed with what I estimate to be about 50 adult guppies, seemingly in good health, and many more fry and juveniles.
Insane. But I didn't say a word of advice to the owner.
Because those fish had somehow slowly adapted to that terrible environment. Cleaning the tank, disturbing it in any way, or moving any of those fish to another tank with clean water, would almost certainly kill them.
I've tried rescuing fish from similar tanks in the past, thinking I was doing a good thing. Even with lengthy drip acclimations, I've lost every one. Clean water was just too much of a shock.
So be prepared for the possibility you will lose these fish if you try to move them to another tank, or attempt a thorough cleaning.
If I wanted to save those fish, I'd clean the tank a bit a time over weeks, letting the fish slowly adapt to the better conditions. Then move them to a real tank. Get rid of that wall-mounted thing, or leave it empty and cover it with a tapestry or something. It sounds too hard to ever maintain properly.
By no means add any new fish to that tank in its current condition.