Does your dad eat cucumber himself? I agree shop produce has a lot of pesticides but you can wash and kill most by blanching them in boiling water. What about growing your own. A grow bag and some cucumber seeds will start you off. A bucket will do nicely for carrots. Mum's love them too. Just ask for vaseline to rub around keep the slugs out.
Carrot pesticides are also removed in peeling them.
Plecs do not need a lot apart from algae as they move slowly and are not using great amounts.
I can say my plec loves Bloodworm and plec wavers as well as cucumber, peas, carrots and courgette.
He has been with me a year and survived.
One thing maybe is that you can overfeed and unlike the ducks in the park too many feeders can upset fish. Enjoy what you can enjoy. It maybe your dad can let you help in decorating / cleaning the tank. Most of all like football they need supporters and mine do well when I tell them my secrets. Well they always look out and my plec winks at me. More than most guys do. Keeps me happy !!! Good luck