Corals not growing and I don’t know why?!?

[GoReefin]

I am using RODI water, but I’ve had some people tell me to use a hydrometer. Would this be a good option?

They have a bunch of different calibration fluids on the market. You want to buy one that is close to what you'll be using in the tank. RODI water isn't what I would recommend.

Here is an example.

 

[Happy Clam]

Under Reef2Reef look up Randy Holmes-Farley and BRS has a lot of knowledgeable videos to get you started.