tops off IMO (not taking jumpers into effect here). the tops will block light. Especially if not cleaned everyday. They trap heat in. the prevent gas exchange>co2 build-up>lower pH>.....
Temp: The average of all the tropical reefs throughout the year .. Average, throughout the year is 84.. In the summer some creep up into the nineties. In the winter, some will drop to the mid 70s. The best recommendation for our average/typical tanks is about 80. I would say 84, but the o2 levels are lower at higher temp and a little less room for error. Some species of anemones will require different temps than others. Some will need more consistent temps, some will accept a widely varying temp. My giganteas seem happiest at around 84-85, but I dont recommend that for the average reef tank. If you are keeping a fish only tank, the 75 that was recommended is a good temp, but corals anemones and such will prefer more natural reef temps.
FWIW, I highly doubt that po4 is your issue here Although they are WAAYY high). I would bet that water flow (which will also greatly effect gas exchange). If your not running a sump and or a good protein skimmer I am bettingh that your gas exchange is low. Good skimmers in a sump set up would be great, but I would at least recommend removing the tops, and adding some water movement via some wide flow power heads (maxi jet mods.. Koriala.. tunze.. seio..) wide flow.
Also the recommendations for RO/DI are another something that should be considered before all else ...