2 Gbrs Dead From Pop Eye In A Week. What Causes This?

[electric yellow]

I have had two fish (GBRs) die in the last two weeks from what I think is Pop eye. Im wondering what the cause could be? There tank is very mature and well maintained. There havent been any new additions in ages. What could cause pop eye? I dont know how to cure it either so advice is much needed!

Tank details...

3ft tank
Running for 2years
Temp: 26degrees
Decor: gravel, lots of silk plants, air stone
external filter
Water change every week of 25-30%
Ph: 7- neutral
Ammonia: 0ppm
Nitrite: 0ppm
NitrAte: 0-2ppm

Stocking:
2x clown loaches
1x gold gourami
1x neon tetra
1x PK betta
1x albino corydora
2x German Blue Rams (1male and female left now)

 

[WILDER]

Popeye is close related to dropsy due to the build up of fluids. Also to much aeration can cause popeye.
Get you a good popeye link
http/www.flippersandfins.net/pop-eye.htm