Why is my whole tank dying :(

[TanyaAD138]

I have a 55 gal large blue gourami tank that housed about 20 Gourami's.

I've noticed them having "stringy poo" sometimes the "string " would be longer then their bodies. I know this is an internal parasite. So i treated the whole tank and to make a long story short, the stringy poo was gone. YAY...

The tank was fine for days, a week almost actually. i haven't put any new fish in or anything. But now, every morning i wake up to a dead fish. No signs of disease this time. Just up and dead. This has been going on now for a week and now what im noticing is, their fins will start to fray a tad then BOOM! DEAD!

What is killing my tank? I've done my water changes, my levels are fine, there's no amonia spikes and my tank is well cycled. I really dont know what to treat them for as i dont know what kind of disease im looking at here.

Anyone have any clue? I just put an order in at Big Al's aquarium for a BOAT LOAD of fish medication (in fact, i think i bought just about everything Seachem carries) because it never fails, they always get sick with the disease i dont have drugs for and by the time i find the drugs, buy the drugs, and get home with the drugs. They're dead.

Im getting discouraged :(

[KingFisher]

Male gouramis can be very aggressive toward one another. They may be killing each other off. Even two males in a 55g will often fight.

[TanyaAD138]

True. But i haven't seen any fighting? Unless they just play nice when mom's around

[Wake Theory]

Frayed fins could also be early finrot, and or some other secondary bacterial infection when they had the internal parasites. You should look into some Maracyn Two perhaps as it would treat just about the entire spectrum of primary and secondary infections in freshwater fish.