Betta fish and water quality diseases

[fleecemist]

I'd double-check the heater too. Sometimes they malfunction and the temp swings without you noticing. A digital thermometer's a cheap way to rule that out.

[celly]

If all else fails, consider a salt bath. Just 1 tsp per gallon of aquarium salt in a quarantine tank for 10 minutes can help with general sluggishness. But don't overdo it—bettas are freshwater fish after all.

[Flynxter]

Honestly, sometimes they just have off days. Mine will lounge around like a couch potato for a day or two, then bounce back like nothing happened. Keep an eye on him but don't stress too much yet.