I made my worst mistake by today. I have a 1000l fish tank with 400 baby gold fish fry. I had a 65mm hose connected to a butterfly valve at the bottom. this whole tank was set up only 1 week ago. I didn't cycle the tank as I didn't think I had to being such a light load (and haven't had any trouble yet). I found about 8 dead the other day and some more the day after so I dont some tests 0 ammonia 0 nitrite 0 nitrate 2deg gh 2 deg kh 5.0ph so without panicing aver the course of the day I got the water adjusted to around 6 maybe a little higher. I done a test that night to see where it was and it came out around 0, 0, 0, 6, 5, 6 - 6.5 This looked pretty good fish swimming around happily. I came in the next morning and ph had dropped off again so off to get more baking soda to fix it again, got it fixed and next morning that same thing happend. while I was cleaning out my kitten kitty litter my friend said to me "I dare you to take a drink" (I was using the fire hose at the time) I asked him what the big deal was and apperently they run fire hoses through a sulpher filter to make the water denser and it is actually poisones to drink. So in a panic I got the garden hose turned it on in the tank and filled it to the top (about 1200l) then oppened the butterfly at the bottom to drain about 400l out (the hose was still going) I didn't open it the whole way as I wanted to go do some work so left it over time) I came back all was fine until I went outside adn saw half the fry had been sucked out and died a horrable death under the napier sun. I now have about 50-100 left and have lernt my lesson (I hope)