I agree with above poster, beantickler. The tank is overstocked and it is dismal news. The only things to do are to decrease the amount of waste and that includes 2 unthinkable options:
1. Rubbermaid tub suggestion (you bigger will need a tank regardless unless the fish are traded in)
2. Trade in suggestion (there are plenty of things comfortable in a 30 gallon)
Regardless of what you chose, a daily or every 2nd day change is mandatory until the fish are moved to a bigger tank.
Trading in is not always possible, some people might not accept trade ins, you could put an add somewhere if you feel it must be done. I dunno what to do. A while ago people gave me doom gloom kaboom stories about how I need a 1000000 gallon tank, I managed to give away a few of my fish. Tanks don't magically appear and they cost MONEY. Not everybody can get one at moment's notice, I have wanted a 150 gallon + tank for 2 years give or take and am not allowed to get one, I also don't have the money.
Here's the deal: Don't buy any animals to add. It is a bad idea. ALL ANIMALS produce waste. NOTHING eats fish poo except bacteria and live plants.
ALSO: snails and plecos are no exception. They eat algae, (which actually absorbs a bit of waste... it thrives in high nutrient water) they don't eat fish waste... They don't "clean up."
Oscars make chow out of snails. I tried adding one a while back when I had an oscar and it vanished so I added another and saw him attack it. I discovered a good treat for him at least.
There is another thing that is bothering me; there is more than one kind of pleco and not all of them get gigantic. The zebra pleco, bristlenose pleco, and those butterfly ones are all relatively small. The rest get pretty big, the most common gets over 1 foot. Those brown-black ones often sold as "suckerfish" or "algae eaters" are the the cheapest ones.