I am a first-time fish owner but I did a lot of research about fishkeeping. I cycled the tank before putting fish in it, it has a heater, filter, and I do 1/5 water change every 2 weeks. Almost exactly 1 year ago I bought 3 harlequin rasbora to put in my 20L fish tank. One died straight away (like the next day of putting him in the tank). I can't say for sure why, but I tested my water and it was perfect, and the other two were fine. A few weeks later I went and bought 3 more fish. They were all fine. Up until a month ago. One of the fish died. I have no idea why. I don't have immediate or easy access to water testing so I haven't got my water tested. Obviously I should. A week later, one of the fish disappeared. I have literally looked everywhere (yes, including in the filter, and the surrounding outside area). I assume he died or got eaten :( now I am starting to worry because a few weeks after the 2nd fish death, another fish died today. One thing I will say is that this fish seemed quite pale leading up to his death. His siblings are brown, and his body became very pale compared to them. So I think maybe he was sick. I know I need to test my water, but here's my problem:
If the water is not high quality, (e.g ammonia), what do I do to make it cleaner? Should I change the water more regularly, etc? If the water IS high quality, how can I figure out why my fish are dying and stop it from happening again? Any other advice would also be appreciated.
Some notes for you: 1. I change 1/5 (4 litres) of the water every 2 weeks. 2. The plants and decorations inside are plastic. 3. The fish are harlequin rasbora. 4. I bought them from a pet shop (Pets at Home), and had them for a year. 5. I use an i25 Marina internal filter. 6. I feed them a couple of broken up flakes of fish food once to twice every day. Sometimes they seem hungry so I give them more and they don't eat it, other times they are ravenous lol.
Re: My fish keep dying and I don't know what to do
I think the issue is almost certainly related to poor water quality. 20 litres is too small to keep fish successfully; the recommended minimum for Harlequins is 60 litres and in a group of at least 6.
In a tank of adequate size and normally stocked it's usual to change 25% of the water every week, so as well as being too small a tank, the water changes haven't been adequate - you would need to be changing more like 50% a week in such a small tank.
It's very important that you test the water with a liquid-based test kit such as the API Master Test Kit (cheaper on eBay/Amazon than in the shops) to find out exactly what's going on. Ideally, you need to upgrade to a larger tank too.