Your pool store is wrong, you do not need to change your sand. I recently bought a new filter. The one I had previously had the same sand in it for 12 seasons and it was fine for many more years. I sold that filter and the guy who is using it is still using that sand.

We need to know your CYA reading. A chlorine level of 1.0-1.5 is too low for a pool that is using dichlor. Dichlor has CYA in it and it builds up over time. Chlorine levels are based on CYA levels. The more CYA you have, the more chlorine you need to keep algae away. I have a feeling this is your problem. Take a look at the Best Guess Chlorine Chart in Janet's signature above for more info about that.