K.R.
Imagine you are new renters, not the old owners. If you were choosing to rent a house with a pool and it was stated in the contract that it would be your responsibility, then wouldn't you just clean the pool so you don't have a mosquito's haven of stale water and algea growing in your backyard? Take all the emotion out of the situation (easier said than done, I know) and do what needs to be done to get through the next six months with as little drama as possible. If you were going to be staying for an extended amount of time, I'd probably put up a fight about it, but it sounds like this guy probably won't budge anyway, and for a mere six months of pool cleaning it's just not worth arguing with the jerk.