Biodiversity and flow of your tank is huge and often overlooked. Everything seems fine as far as your parameters. Many reefers run at 1.026 salinity so i highly doubt that is the issue. Unfortunately your tank just needs more time to mature. Make sure you have good live rock in there. You can even buy some commercial bacteria additive to help give it a little bit of a boost (but dont rely solely on that). Just give it time and youll be ok. Id suggest not adding any more corals unless they are the hardy type. 4mo old is definitly a bit early to keep acros healthy. Just time and patience is all you need. Your doing great with everything else!