hi bro, may i just suggest that mandarin goby needs a much bigger tank to survive... had my fair share when i first started out. didnt do research....went LFS and was told it was easy to care for.... then after 6mths of trying to feed it and adding of copepods...it starved and died. now i stop at 3 small fishes in my LPS dominated RSM 130 Tank.
since u're going in the direction of very small tanks... y not use an IQ3 or IQ5 tank since your water parameters are at +-0ppm nitrate? a pump, a good LED light, a cooling fan, very strict routine daily water change/top up and u're good to go. just a thought...
neways, too much liverock in a small tank will contribute to bad water circulation = algae.
lastly, with the corals u have, water changes are sufficient. as for supplement, coral food once a week is fine. other than that, nothing else is needed.
best not to disturb the corals and water all the time.
Alkalinity should be around 9-12dkh. Most fresh salt mix will have around 12dkh.
pH should be around 8.2 to 8.4 at 26 degrees celsius.
dun worry about calcium and magnesium for now. water changes will replenish all these.
hope ur tank will recover and come out to be one of the best bro