Your tank is far too small for starters, in terms of swimming and growing space, and also with suh a small amount of water, their waste will buid up unless you do big water changes every couple of days even with a filter.
You're going to stunt their growth if you keep them in there, if they haven't grown in all the tikme you've had them then you're stunting them already.
Get a big clear plastic storage tub , the biggest you can get/have space for , and set that up in the same way as a tank with dechlorinated water . It will give them far more space to grow. Use a bog standard sponge filter, or a Fluval 1/2 or something for them. They don't need a heater as they're coldwater fish.
You don't have to use any gravel or anything if you want to keep the tub easy to clean, but perhaps a thin layer of sand or fine gravel if you like? Put some cheap fake plants in there for them to hide in as well. A completely bare tank is so desolate.
Do 25-50% water changes once or twice a week.
Where did you get the gudgeon by the way? If you got them from the wild then they probably will be reluctant to accept flake foods and suchlike. Have you tried live foods like bloodworm and Daphnia? also tiny baby earthworms are good. It's live foods such as those that they'd eat in the wild and they're a bit more likely to eat those than dry foods.
