Not to ask the obvious question but have you fed them?
Did you jus feed them, as in, right now before you looked at them My fish all look bloated after a healthy meal.
Size of tank in gallons please?
How many fish and which type?
Water stats in ammonia, nitrite, nitrate, and ph?
To many dried foods can lead to bloating.
Are the scales sticking out?
Feed some peas. Don't feed any dried foods for a few days.
Do you feed any frozen foods?
How long has the tank been set up?
I would introduce some frozen foods into there diet like brime shrimp, bloodworms, daphnia. Don't over feed
blood worms as they can also cause bloating.
Also green veg in there diet like peas, sprouts, broc, spinach, cabbage.
Fish do sometimes turn there noses up at peas. Keep trying.
Add some frozen garden peas to boiling water for a few minutes,
Let the peas cool down. Then pop the pea out of its shell. Chop into tiny pieces and feed to your fish.
Remove uneaten peas after a few hours.
Is your tank 50 gallon, or 50 litres?