whats going on?

[gonnelro]

I agree, probably ammonia spikes because you added all those fish. Id rinse the filter media out in water from the tank as well in order to preserve the BB. And when you add the salt, Id add it slowly and dilute it before you add it to the tank. Not that it makes a big difference, but I think thats teh best way to do it. And listen to everybody else, the fact that youve kept fish alive for two years does not mean at all that you dont need to test the water parameters. In fact, when adding fish like you have, you should definetly be testing to see if the bioload is too much. Your filters need to cycle, and time is needed to allow BB to grow over your bio-media. Trust me, test, test, test, and Id do bigger water changes than the 30% youre doing, although it depends on the bioload. Why not just do 50%?

 

[badfish]

Even if your water has been fine for two years you have changed the stock level and you haven't changed your water change schedule you have to adjust to the stock level. water testing is necessary when you make changes what works for one oscar may not work for a community, if its not your water then you may have introduced a parasite, did you quarenten your fish?