Try checking the PH of the water you are starting out with .... in a bucket, then go along to the next step - put whatever gravel you use in that bucket of water and check the PH next day (try to make it 12-24 hours in-between steps) - I've had to do this before becuz I couldn't figger out what the cause was - - one step at a time - until you find your culprit! In the meantime, if your fish aren't suffering, leave them in the original tank whilst you go thru the steps to find out the reasons for your problem in a separate container. bsfman may have already answered your question - don't be in too much of a rush to give them fresh water, your original water may just be going thru a weird cycling event - (personal experience )

Just my 2 centavos .......