A few issues here and the main one is tank size, 60 litre (15gal) is way to small for Oscars, one alone needs 55gal (210 litre)
Internal filters are pretty useless as a main filter for Oscars too as they produce so much waste.
Water changes in a tank that size with 2 Oscars would need to be daily at about 80% per day
Water temp should be around 26c so your a bit low there too.
Food, They require a mixed diet with some veg matter and it is never a good idea to feed feeders as they can carry parasites and disease.
I have never really been a fish person but my brother in law moved out and left the poor fish behind in a filthy tank so i cleaned it and have looked after them since, they seemed to be so healthy and grew really quickly once there home was sorted out.
never a good way to have to start fishkeeping, so good on you for taking care of them the best you could.
If you intend to keep these fish then I suggest you really need to upgrade to a tank of around 90gal (340litre) and canister filtration with a minimum turnover of tank vol x 5. If you can't manage this then it would be best to rehome them as the care will become a task more than enjoyment long term. Oscars can live 15+ years, so a proper tank/filtration is a must.