We have 2X Round Goldfish tanks - 1 gallon capacity. We are now on fish 6 & 7 and No 6 has problems. We have had the water tested & OK, we are feeding the correct amount, we are putting oxygen tablets in the water, we are using "stress coat" when we feel it necessary, we are changing 10% of water weekly. The fish only seem to last a week or so. Last time (12 days ago) we cleaned the tanks with ordinary water, filled and conditioned the water for 2 or 3 days (tapsafe) and then introduced the new fish. No 6 was happy and lively until yesterday - Any ideas ???