Well, low pH is actually pretty normal for an older system that was never dosed with calc or alk. Since you guys are so into the researching aspect, let me give you some reef chemistry knowledge to chew on
. Here's a
basic guide to calcium, alkalinity, and magnesium and their relationship in reefkeeping. Since your surface agitation is pretty good I'd say your low pH is probably the cause of low calc/alk/mg. Not the end of the world and it can be corrected if done properly. Once you understand the relationship between those 3 elements, test your water to see where you're at. If you are low on calc/alk, check out this
easy two part additive guide for longterm slow dosing to bring things back up to where they need to be.
My pH has slowly crept down as my tank has aged and my coral population increased (consequently draining calc/alk). I've just recently started dosing with the method I linked to slowly bring things back up to where I want them.
As for cleanup crew. Stick to smaller snails like nassarius, cerith, and trochus for the bulk of your crew. Then a couple bigger guys like a shrimp or two and maybe some scarlet hermits. I'd stay away from blueleg or zebra hermits since they will fight amongst each other and kill snails. I'd also stay away from trubo snails which cant tolerate summer heat and astrea snails which cant flip themselves over when they invariably fall in a place where they cant right themselves. You could also consider a brittlestar but keep in mind that in 3-5 years it will likely outgrow your tank. They live in small areas naturally, but can get large in captivity with no predation
Water tests should never be performed immediately after a water change. Allow at least an hour for things to mix and settle. When performing nitrate tests, make sure you follow the directions EXACTLY. Some bottles need to be shaken for certain times and never use your finger on the test tube, skews the reading a LOT. Was the nitrate kit you bought specifically for saltwater? Cause the saltwater and freshwater kits have way different color tables.
RO water cannot be tested for nitrates cause of its low solute count. The only way to test RO water is with a TDS meter (total dissolved solids)