Hi Byron Thanks for your helpful information. I syphoned the gravel last night and did a partial water change. The nitrates have gone down to 100. The water in the tank looks very clear today. I made a mistake, our tank is 28 litres. We would like to get a bigger tank and re-house our fish. What size would you recommend? We haven't got an enormous lot of room in our small house. Thanks again, Dave Layton.