I have a fantail goldfish in a 26 litre tank with a good filter and oxygen supply.
For weeks now he has been just sitting on the bottom of the tank.
I have been doing water changes I have checked the water parameters and everything is within range.
He had recently got worse and had gone off his good and now spends all of his time lay on the bottom.
His colouring is still good and there are no white spots or signs of any other issues. But I have been using a general fish treatment just in case for 2 weeks and he hasn’t changed.
I use a mix of pellet and flake food.
Any advice about how to help him would be amazing! Thank you in advance.

Hi
How long have you had the tank & fish? Did you fishless cycle the tank before putting the fish in? Would you post actual test results for ammonia, nitrite and nitrate, please?

The main cause of his problem is that the tank is far too small for a goldfish - they grow to be very large fish (8 inches long excluding tail) and have very bulky bodies. They also produce a lot of waste which accumulates in small tanks and impairs their growth and damages health. This article explains what he needs and why, but the answers to the questions above may help us recommend immediate steps to help him. You are going to need a much larger tank of around 180 litres with very good filtration though.

Unfortunately I cannot tell you the exact parameters as I will not be back home until Saturday now (mum will be keeping an eye on the fish) but I checked them yesterday and they were all within the stated normal range.
I have had him for about 6 months and he is 3-4inches now.
The tank was cycled before I put him in and I try to do little and often water changes so I don’t mess with the water balances too much.
I was advised in the shop to buy him a bigger tank as he grows which I have recently done and got a 65 litre tank but with him not being well I didn’t want to move him.
Do you think I should start cycling the new tank incase it is the tank causing the issue?

I would move him to the new tank a.s.a.p You don't need to cycle it: just take the media (sponges etc) out of the existing filter and put it in the new tank's filter. That transfers all the beneficial bacteria and you'll have an instantly cycled filter. Water changes should be at the least 25% of the water volume every week. If nitrates rise too high, too quickly (fancies need nitrate kept below 20ppm), then increase the amount of water changed or the frequency of changes to keep it under control. Nitrates above 20ppm often cause fancies to have buoyancy issues.

Thats great thank you for the advice! I shall do that ASAP!!