got my daughter a small 25L tank for her b-day in september, put 3 goldfish in it. was fine for about 3 weeks, did the same for it as my big tropical tank, gravel vac- weekly water change. goldfish started dying off 1 by 1 & i kept buying more & the same thing, they would last a couple of weeks & then die aswell. took most of the water out & replaced with water from my tropical & small amount of fresh water ( told by lady at lsf to do this ) things where fine for a couple of weeks then started again. i gave up with goldfish, completely scrubbed the tank put water from my other tank in & put 5 guppies & 1 borneo sucker in ( with a heater of course). now 1 dead sucker ( think i may have crushed him but not sure), 1 dead guppy & another one about to die, what the hell is wrong?? could it be the tank, can the wrong silicon (sp) been used? dodgy rocks? all had been well cleaned. there was one goldfish left & i put it out in the pond with the others & yip that died too. so i have no idea what to do or whats going wrong