When I worked at 'A Northern University' in the early 90's the biology dept kept a number of fish and crabs etc. for research purposes in fibreglass tanks as seen in the aquaculture/fish farming industry.
What was unusual was that the adjacent corridor had a number of large display tanks set into the wall, just as you would find in a open-to-the-public type aquarium. [However, the corridor was restricted to University staff only]. The largest was about 12' x 4' x 4' but was not in use, and was full of rather stagnant water. The other two were about half or a third the size - one marine with an octopus that would occasionally get fed one of the crabs; the other freshwater with a few roach, the biggest of which was well over a pound in weight!
I decided to renovate the largest tank and move the roach into it. Over several days, I drained the water, shovelled out and washed all the gravel, scrubbed the sides etc. On one occasion when standing in the tank, something appeared to move below the gravel. I mentioned it to one of the lab technicians who said - that'll probably be the eel, it's been there for years - we never actually feed it! Not sure how true some of that was, but it certainly was an eel of about 12oz - 1lb!
Anyway, having renovated the tank, incl putting in a huge chunk of bogwood that al ocal farmer had pulled out of one of his fields having hit it with a plough (it took about two weeks of soaking to get rid of the peat staining), I asked persmission from a local angling club (of which I was a member) to take a few fish from their lake for re-stocking.
So in the end we had the eel, the roach, 4 or 5 skimmers about 3-4oz each, a similar number and size of rudd, and a group of about 6-7 small (2oz) perch. The last were my favourites - full of energy and attitude despite being the smallest in the tank. Fed the lot with maggots daily!
One day some of the fish developed a fungus, so we treated with malachite green (a copper based antifungal - previously used extensively in trout farms - now banned). I probably overdid the dose - you couldn't see more than an inch into the water and I was a bit worried! The next day when all had settled, everything had survived apart from ALL the perch - dead! Probably a rather crude, unintentional experiment of the relative susceptibility of coarse fish to copper poisoning.
Left the dept a year or so later - no idea if anyone else took on the upkeep!
Ian