Zebras with tail/fins shrinking?

[YSR50]

Posted this on another board and the thought was that maybe the Alpha chewed on them or possibly fin rot. Honestly they don't look beaten up or in bad shape, just almost like they've shrunk. Just thought I'd check in here. Thinking of selling these guys but don't want to list sick fish. Also, dumb question but… the fins/tails should grow back, right? I have had L134s get chewed up in the past and the fins/tails have always bounced back pretty well.

A general description of the symptoms:
I've noticed in the last few months that 2 out of 3 of my 3+ year old zebras have very little tail left. Their fins seem smaller also. The dorsal fins aren't bad though. My alpha male zebra that's the same age (as well as the other tank mates) is mostly fine. I don't check the water parameters very often and I let the eheim go for too long this past time so I was wondering if this looks like a water condition issue? Not sure why it would only effect 2 of my fish though. Don't think the tank mates would be the problem.

Size of tank and other tank mates:
15 gallon (24x12x12") with 2 sterbai and 3 dwarf neons. Setup has remained unchanged for almost 3 years.
No lights on the tank, just gets light from surrounding tanks.

Water parameters tested with Easy Strips:
Ammonia: 0
Nitrite: 0
Nitrate: 40
PH: 7
KH: 60
Gh: 150

Filtration and water change regime:
Eheims 2028 pro 2 cleaned about twice a year, Emperor 280 (media changed every month), Eheim aqua ball and 6" air stone
Water changes about 2-3 times a week, 2-3 gallons of tap each time treated with API
Once in a while I'll use a gallon or two of RO water that has almonds leaves soaking

Diet consists of mainly nls Thera + a, and flake for the others in the tank. With the occasional shrimp pellets, frozen brine shrimp and frozen blood worms

Temperature of tank water is about 85

My alpha, wouldn't leave his cave, but he looks pretty much ok.

[jsalas623]

They should grow back! Maybe putting these guys in another tank that way their stress levels will get back to normal. don't want them getting sick due to stress.

[TwoTankAmin]

Looks like aggression to me. Try a bigger tank?