I'm going to guess it's not a O2 issue. 20g with 3 small fish and you have surface movement, not O2. Easy way to test, siphon 5g into a bucket, then dump it back in the tank. If you don't see noticeable improvement immediately, it's not O2.
All fish breathing heavy and not eating sounds more like disease to me. Or contaminants in the tap water.
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Brad
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