Paulito, your tank is too nuevacito. It just doesn't have a mature biome to support SPS like acropora. Personally, you should wait 9 months to a year. For now, do what the other reefers have suggested - get in the habit of maintaining water quality.
If you could only test one thing, test alk (assuming that you already keep your temp and salinity stable). If you don't have a lot of stony corals, your alk consumption will be low. You can get away with weekly or bi-weekly water changes. When you add LPS or have a lot of coralline growth, your alk demand will increase. That's where the testing comes in and knowing how much to dose/supplement.
As far as STN/RTN on your acros, most likely due to tank immaturity.
If you could only test one thing, test alk (assuming that you already keep your temp and salinity stable). If you don't have a lot of stony corals, your alk consumption will be low. You can get away with weekly or bi-weekly water changes. When you add LPS or have a lot of coralline growth, your alk demand will increase. That's where the testing comes in and knowing how much to dose/supplement.
As far as STN/RTN on your acros, most likely due to tank immaturity.