Hey capt. =)
I have a similar situation with my stepdad and his 5 inch long shubunkin goldfish he keeps in a 55 gallon tank with a slightly smaller common goldie. I've been trying to coax him into getting a bigger tank for them, but he's not having any of it. Now, every time I go over (about once a week), I test the water and do a water change without even asking, lol. He gets a little miffed, then shrugs his shoulders and gets over it. Would that work with your dad? It sounds like he really doesn't care that much about the tank or it's tenants... do you think he would get angry if you just went and did the water changes without sneaking? I mean, hey... you're the one doing the work, not him.
Good luck with that situation, though. That would drive me nuts, as well. Why even bother having any animals if you don't care about their well-being? Makes no sense.