If you're washing the outside glass with any cleaning products this can cause fish to die off and weird substances to float on your water. because glass is porous, the cleaning solution will creep through the pores, up, over and into your tank water. Its not a problem right away but after a while it will build up until one day poof, no more fish. Or, like everyone else suggests. its just a high protein problem leading to high nitrates and nitrites.
#1 way to salve most established fish tank problems is water changes!
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