Hi again, sorry I really should have posted a bit more information from the water testing I did. I set both tanks up at the beginning of this year and at that stage we were using water from our bore. We changed over to the rainwater tank approx 6 weeks ago. I do 20% water changes every week and I test the water approx 2 - 3 weekly. My PH is 7.0 in both tanks and I have 0 ammonia and o nitrites. These readings are the same since changing from the bore to the tank. I wouldnt have thought changing would cause us any trouble, but since the change, any new fish that we add which happen to have been guppies seem very stressed, red in the gills and sit bouncing on the surface of the water. Last night I lost guppies from each tank, whereas all my established fish from both tanks remain healthy and happy. I am at a loss and just wondered if anyone had any thoughts on whether or not the change of water supply could have an effect, even though I would not have thought it would. They are both community tanks with a pleco and bristlenose each, neons, glowlights, black phantom tetras and a dwarf gourami. One is 180 litres and the other is 125 litre. Thanks again