Best Way to "Purify" an infested tank

[ZeroOne]

So. I wrote before about a problem before in my 40g where my golden gourami's keep dying. Well today I lost the last one, red spots stopped swimming fell sideways, couldn't nurse it back, died.

I ask today what my options are to "cleanse" my tank. All other species in the tank are fine, except a seemingly new population of pond snails (can deal with). But there's gotta be something in the water that's killing the gourami's.

Whats the best way to clean the tank, I'm thinking 10-15% water every day or every other. Should I do more or less water, more or less often? The problem has got to be biological, the water has been tested, retested over and over and always comes out fine. So opinions on cleaning it out would be great! thanks!

 

[Lupin]

What other fish do you have that are fine?
Are all these gouramis from the same source?

 

[blacktarotannis]

What do you have running on your tank?

 

[gomezladdams]

If you dont know the pathogen,the only way to cleanse the tank is bleach everything and start over.

 

[Mac.]

If you dont know the pathogen,the only way to cleanse the tank is bleach everything and start over.

 

[Dark Jester]

Are all these gouramis from the same source?

That would be my question too. If you got them from the same place, then they may have all had the same disease and just succumbed to it at different times. Even if you got them from different sources, but there was always at least 1 in there from the previous batch, it's possible some disease was communicated between them.

If the other inhabitants are fine, I'd let it go for a week or two with no gourami in there, then get 1 more from a source you haven't used yet. See if it makes it.

If it falls ill with the same symptoms and dies, then you know it's something in the tank. Probably have to bleach everything and start over if you really want gourami. I wouldn't even keep the same fish with any new gourami at that point. They may be carriers for whatever disease it is, but maybe resistant to it themselves.

If it lives then it must have just been something wrong with the others and you're probably safe adding more.