Start by carrying out the large water changes rabbut mentioned - by all means, but . . . errrrrm . . . 75-90% is too much. Once there's fish in the tank, you shoul aim to change no more than 50%, because the change in water paramenters (namely pH, nitrate and temperature) can shock the fish.
TetraLinz, may i ask why you think 50% is the biggest water change advisable?
When dealing with ammonia and nitrite, the single most important thing is to remove these toxins from the water. pH will generally be the same in the tap as in the tank unless the tank has gone a long time without maintenance, temperature is easy enough to control and a drop in nitrate will benefit the fish more than shock them.
Many members on here (myself included) advocate changes of up to 100% in these circumstances and it can quite literally mean the difference between life and death for the fish.
Assuming the parameters of the replacement water are roughly the same as the waste water, the water change will have no negative effect on the fish, and this will be the case in the majority of circumstances.
You are correct that large swings in parameters, even for the better, can be detrimental to fish health, but many people underestimate their fish's resiliance. The shock of a slight change in temperature or nitrate level will almost always be far out-weighed by the benefit of the large water change (removing ammonia and nitrite).
I'd like to hear your side of the discussion as you seem to hold unfortunately popular views which contribute to the demise of millions of aquarium fish each year through ammonia and nitrite poisoning because the fishkeeper has been ill-informed that large water changes are somehow worse for the fish than being poisoned.
There are exceptions where large water changes are not adviseable, but this usually applies to mature tanks which have gone without maintenance for a long time, not new set-ups as is the case here.
It is essential to stress the importance of keeping pH, hardness and temperature relatively stable during water changes, but this can usually be done with relative ease. I would suggest that to advise no more than 50% should be changed at any time is, as Miss Wiggle says, old-style fishkeeping and holds no weight in today's hobby.
Bblou, i advise you to perform a 75% water change to bring down the nitrite to a manageable level and then continue to perform as many (and as large) water changes as required to keep both levels below 0.25 at all times. Media donation will definitely help as long as you can detect ammonia or nitrite in the water. I'll be pleased to explain it further for you, if you like. Good luck!
Cheers
BTT
thanks again
