Within the last 2 months have you done a large or 100% water change and if so when?
Have you tested the water recently for levels of ammonia, nitrites and nitrates and if so what were the exact results?
Thanks for the information but there's still some unfilled blanks but you've given me some important information otherwise.
You didn't mention a test result for ammonia so please look into that as it's very important. Ammonia should always be at -0- in an established tank. It's deadly to fish.
You stated that you did a large water change but didn't say exactly how much as there can be a bid difference between a 50%, 75% and 100% water change.
Ok the important and pertinent information you supplied which has much bearing on this case is the nitrite result which=.5. Basically that is the cause of the problem as nitrite like its predecessor ammonia is lethal to fish. Nitrite should ideally always be at -0-.
I suspect when you did the large water change much of the "good" bacteria which breaks down ammonia and nitrites was eliminated.
What needs to be done is keep nitrites under control. It might take some time for nitrites to be completely gone as the "good" bacteria will need time to form again.
This is actually called cycling and in your tank's case a "mini-cycle" would probably be the appropriate description. Click on link below for a brief overview of cycling in case you're not aware of what it is.
What you'll need to do is frequent 20%-25% partial water changes and test the water every 2-3 days for ammonia and nitrite levels. If they begin to rise a partial water change would become necessary.
To help things along and to not contribute to ammonia/nitrite growth it would be advisable to not overfeed the fish as overfeeding causes excess fish waste which in turn fuels ammonia/nitrite growth. Feed the fish only a small pinch of food once a day or even better once every other day.
Please keep in mind there is no quick fix, aside from changing all the water, to eliminating ammonia and nitrites.
Good luck and let me know if you have any questions.
the water change I did was between 60-75%. I was unable to do a 100% change because the plecostomus a) stays inside of a castle, b) is too large for me to put in something else and c) he scares me. lol. I did not replace the filters in the filter on the back of the tank but I did replace the undergravel filters at that time. (2 of which are charcoal and 2 are amonia removing ones)
Also, I took out as much of the old gravel as I could get and replaced it with new gravel. I took out most of the decor (other than the castle where the plecostomus lives) and replaced that as well. The reason I did not include amonia results is the test I have does not measure the amonia levels. However, the AmQuel Plus removes Nitrate, Nitrite, Amonia, Chlorine and Chloramines.
It's safe to say that a water change of 60%-75% in conjunction with the replacement of the underground filter and a large portion of gravel was significantly enough of a change to have eliminated the majority of the "good" bacteria which breaks down ammonia and nitrites thus the existance of at least nitrites in the water.
Ok thanks for the clarification on the ammonia testing but at this point in time I'd suggest getting and using an ammonia test kit as if it is also present then you're up against something even more serious.
As for Amquel Plus--it is only a temporary fix and will not permanately eliminate ammonia and nitrites. Ironically ammonia and nitrites are needed in order for the good bacteria to form. The only way around this whole thing is to do frequent and large water changes which would halt the formation of ammonia thus never permitting the tank to cycle. The tough part of this is that you'd be doing some very very large water changes
every few days--not a pleasant way to enjoy one's aquarium.
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