I would suggest testing your water quality with a test kit for starters to make sure that your ammonia and nitrite levels are low. If those check out fine, I would then check your oxygen levels by using an oxygen test kit. It’s always a very good idea to use an air pump over the winter months so that your dissolved oxygen levels are always high. It also does not hurt to use an enzyme to control dead leaves and waste over the winter months and I would suggest using a product like UltraClear Oxy.
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