Hi, I am new to this forum and very new to fish keeping and I've had a bit of nightmare, so I'd be very grateful for any advice. About 4 months ago my mum turned up with a present for my 4 year old daughter, a 30L fish tank, with filter, light, water conditioner solutions, 2 goldfish (common) and a few liters of aquarium water from the pet shop. In went the fish and my daughter loves them. Not knowing much about fish I did a bit of reading and learnt that perhaps that wasn't the best way to start, still, the fish were happy and through trial and error I got the right balance between water changes and feedings and they settled in, all seemed well until a couple of weeks ago when the tank suddenly had a bit of an algae bloom, it really went very green so I reduced the amount of food to just 3-4 flakes a day and the light to 8 hours (from 12). The algae continued to bloom and I could hardly see through the tank by the end of that week. We were due to go on holiday so I did another 50% water change and left them a block of holiday food. We were gone for 7 days and when we came back the tank was a green soup of a mess but the fish were swimming around happily. I removed the food straight away, tested the water for ammonia which was 0. The next day (sunday) I did a 50 % water change, cleaned out the gravel and ornaments, filter etc... and fed the fish their usual 4 flakes. I also had a chat with the man at the pet shop about the algae and he said I should reduce the light to only 4 hours for a week and gradually increase it until I found a happy medium. So I've been doing that. Fish were fine on monday, but on tuesday morning, the smaller of the two was swimming uncontrollably on its side. A quick search of the internet suggested swim bladder. I tested the water and nitrate was about 50 and nitrite about 0.5, still very green, so I didn't feed them that day, did a 25% water change, then that evening I fed the sick fish a boiled pea without the shell. Then yesterday morning I found him lying on the bottom of the tank, still alive but it's body bent. Moving every now and then but can't really swim. Did a 10-15% water change, nitrate measured 25, water still very green, also changed the foam on the filter as it had been a month. Today his body had uncurled but he's still not moving much. I hate seeing the poor little animal like that there is so much conflicting advice on the internet I don't know what to do for the best. From what I've read he's not likely to recover but if there's anything at all that might help, I'd be grateful for the advice. Thanks in advance!
Common goldfish (single tail) are not suitable for aquaria and certainly not for a tank of 30 litres. Fancy goldfish are suitable for aquaria but only of a significant size and Im talking about 100L for the 1st fish and then and additional 40L or so per fish on top of that. For a 30 litre tank you are really only looking at housing shrimp and snails and no fish.
Your best bet is to go to somewhere like B&Q / homebase and buy one of those plastic storage tubs thats at least 100 litres in size and move them in there, you will need to add the filter, maybe a heater set fairly low (around 20-22 degrees C) and an air pump to aerate the water. You will most likely have to do a Fish In Cycle which is pretty bad for the fish but not a lot you can do about that now. there will be some articles on cycling a tank on the forum.
You will have to do large daily water changes of at least 50% of tank volume to keep on top of the ammonia/nitrite build up as these are toxic to fish.