I agree with k30a that you need to do frequent water changes.(not just
"topping-off")
I don't know your level of knowledge of water chemistry, but, I would
recommend monitoring water quality closely.
I saw a nice "dip-strip" pond water test kit at Lowe's for about $15.
It would be a nice tool for learning about the nitrogen cycle, and,
personally, I find it very interesting.
Plus, it would give you a better insight into potential problem
diagnosis/prevention.

Doug
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