Betta fins suddenly shredded

Ack! I have 2 bettas in a divided 2.5 gal tank with a Nano Filter. I
woke up this morning and both bettas' fins are in tatters. I'm talking
total, heartbreaking shreds. Both were in perfect shape last night. I
immediately tested the water, expecting to see ammonia but it was 0/0/5
ammonia/nitrite/nitrate as usual. I changed 60% of the water and added
extra AmQuel anyway. The tank has been cycled and stable for about 3
months and gets weekly 50% water changes. It also has a lot of java
moss that would tend to soak up an ammonia spike.

I also cleaned the filter, which gets some sunlight, and the inside was
coated with blue-green algae. I've heard of toxic cyanobacteria and was
wondering whether cyanobacteria toxins can cause fin problems and
nothing else? I'm grasping at straws.

Or did one of my bettas jump the 2 cm of divider that's not covered by
glass, neatly dodging the filter intake, fight his tankmate, and jump
back?!? There is no other way around or through the divider. A fight
fits the best but seems improbable. Both fish are eating, behaving
pretty normally, and nobody's talking.

What do you all think and does anyone have experience healing bettas'
fins after a fight? I always use very clean water and more frequent
feedings for fin damage, but I've never seen betta fins this badly
damaged. Are there any other betta specific tips or tricks?

--
Elaine T __
http://eethomp.com/fish.html '__
rec.aquaria.* FAQ http://faq.thekrib.com