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[chrisbassist]

Each 50% change will halve the amounts. If they don't seem to be doing anything then it's probably actually too high for the test to read.

Fish in cycles can be hard work with the amount of changes you end up doing. But be persistant. Do another change, do another test, if it's still really high, do another change as soon as you can (but at least give the fish a few hours to recover) keep doing that until you have safer levels of all toxins. Then you can slow the changes just keeping them under 0.25 Good luck, hope your fish are doing ok

 

[Tolak]

The problem involving larger than 50% water changes is that often an aquarium has not had larger or frequent water changes before this, creating a situation where the replacement water has vastly differing parameters that what is in the tank. Seeing as you have been doing some 50% water changes this would not be the case. As long as the fish have an inch or so of water to swim in, depending on their size, larger water changes will not be a problem.

Most of my water changes are in the 50% to 75% range, at times larger depending on the stocking & what I'm trying to accomplish.

 

[OldMan47]

I must agree with Tolak. If I have water problems on a newly set up tank, I leave just enough water to keep the fish from drying out and change the rest with carefully temperature matched and dechlorinated water.
I did this on a tank that contained a poorly matured filtered and was showing nitrites. Each day for 3 days running I changed almost 9 gallons in a 10 gallon tank. Since it had gravel in the tank, the water left behind was less than n1/2 gallon. Each time I did the water change, the fish looked much better afterward. By the time I had done that 3 times, the filter had caught up with the biological load and I was able to back away from the huge water changes.