I don't know what you mean by saying acid. If you had acid in your tank all your fish would be dead! Do you mean the element ammonia? I'd say the ammonia increase in your water is due to there being so many fish in it. The ammonia will lead to a higher nitrite level and a higher nitrate level. Only the nitrate can be removed through the water changes, and it's the ammonia you need to worry about. You should add in some ammolock, it won't remove the ammonia but instead convert it into something more healthy for your fish. To get the ammonia levels down you will have to remove some fish, otherwise you are putting your fish at lot of risk. I'd say for your tank size, it is big enough for just the 5 cory's, the molly and the platy. If you have a big shool of guppies in there, then the increase in the fish waste is the result of the spiked level as their waste s converted into ammonia. Pleco's need a minimum tank size of 15 gallons on their own to live in, as they can grow up to 6 inches at adult size, so I would find your pleco a new home, and find a new home for all of your guppies.
