RE: Water Quality
If you are running a reef tank especially an SPS tank I would suggest checking calcium and alkalinity every week and magnesium at least once a month. The rest of them will depend on how mature your system is. I rarely check the others if everything is looking good. IMO phosphate test kits are not worth the effort. Whether you are having a nuisance algae problem or not is probably the best test there is for phosphate. If your alkalinity is where it should be in a reef tank then you probably don't need to test for pH. Ammonia and nitrite should be test at least weekly when a system is cycling. Nitrate should be tested at least once a month.
You can get a combo ammonia, nitrate, nitrite, pH test kit for a lot less then the individual Salifert kits. You won't need most of them often anyway.
Gary
125 SPS, 75 gal. LPS/softie reef, 9 gal. Nano