I have that same tank but in white, for quarantine purposes. A larger tank is needed for sure. I think your parents need to learn that when they provide a special needs child (or any child) with a pet, they need to take responsibility for the pet when a child cannot or has grown tired of the pet. Maybe you could make up a a few printed pages on general Carr and supplies for them to have on hand?
And yes, at least a 50% change and a 100% change once a week. Maybe your parents could take out 50% of the water while you're not there and replace it with treated water you leave them in a container?? It would be room temp and ready to add. I do this for my fiance when I leave the house for a few days.