"Reel McKoi" wrote in
:

Do you have a flow through system to keep nitrates below 20mg per liter?

Nope. Just 30 % or so weekly changes with clean (no nitrates or other
nitrogen stuff) tapwater, quite a lot of living plants and most important:
not too many fish and no overfeeding.

The tank is a 55g ... I recently added 8 Neons,
2 small pearl graumies, 5 platys and the two corys (one died).
There were several ottos, 2 clown plecos and a reg pleco in the tank.

Anything called "regular pleco" is far too big for a 55g. Well, sometimes
bristlenose plecos (Ancistrus) are sold as "common plecos", and they are ok
in such tanks, but most often the "common plecos" are sailfin plecos
(Pterygoplichthys or whatever their current genus name is) or Liposarcus
plecos (again, the genus name has probably changed recently). They get half
a meter long (over 1 foot) and they need HUGE tanks to be able to live
happily.

Other than that one fish, the tank doesn't sound overcrowded although it is
a bad idea to add new fish to a tank with high nitrate level, and it is
also a bad idea to add lots of new fish at the same time as the bacteria
doesn't multiply that quickly and you can kill your fish with ammonia or
nitrite even if the tank is old (like yours is).

Cory cats are schooling fish, and it is not right to keep them in groups
less than 5 individuals of same species.

Do a 50 % water change (not 90 %, it is not a good idea to change water
quality too much too quickly) and test nitrate immediately after that. Wait
for 5 days or so and feed the tank normally and test nitrate again. If it
had gone up again, you have clearly too many fish and/or feed far too much.
In that latter case it would be good to test for nitrite too, just in case.
And test with a drop test, if possible, not with paper slips as they are
not that accurate.

Liisa