TDS meter and lighting question

[Parnold]

My TDS reading is at 53 on my top off water. Does this mean I should change my RO filters. The filters are only a couple months old. At what TDS reading do people usually change them.

I bought 400 watt MH lighting when starting this tank, it is only 22" deep, is this too much light, they are 14" off the surface with no cover on the tank.. I dont seem to be having alot of success with corals. I do have to add a ALK buffer to keep it from going below 8 dKH but other than that my water seems really stable.

Thanks for any response