Another thing you can do is add hose to the system. As you have found just sitting there it will warm up to the outside temp. If you just add another 50 or 100 feet it will be warmer a lot longer. And if you buy black hose it can get very warm or even to hot in some cases

For our kids outside pool (100 gallon play pool) I have a waterfall pump attached to 200 feet of black hose. On a sunny 75F day I can get the pool to 95F!