no water changesIf you opened the canister filter now, how loaded with collected detritus or "dirty" would you consider it to be?
You should have a sufficient SSA (Specific Surface Area) in the canister filter to handle your present fish load, so I'm wondering if the canister filter contains an excess amount of decaying organic matter that is contributing to the rise in the ammonia level.
And to clarify, you are not performing any water changes, correct?
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I would say the it's conservatively a 3, being 10 the worst
here's a pic, do you agree?