Betta tail fin turning dark blue?

Dec 9, 2019
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I've had a veiltail betta in a heated (78C), sponge filtered, planted 3.8 gallon tank for 11 months now. I do 30% weekly water changes and alternate vacuuming the gravel and squeezing the sponge filter. The betta's battled fin rot over the summer due to temperature fluctuations but has always regrown the tissue quickly and has otherwise been healthy and very active.

The last few months I've noticed the edge of his tail develop a dark purple/blue that is slowly spreading and is curling up along the edge. Since developing this his tail has not been grown back as it usually does after having fin rot there.

Attached are the before and after photos, first in April this year when his tail was longest and just now in December. The only thing I can think of is ammonia burn, though the test strips I've used say the water chemistry seems fine. Any insight is appreciated.


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