Heater advice GHL profilux mini

Hello!
I have been running a saltwater tank for about 4 years now. I have had many ups and downs through the years. I started with a small 20g with 2 rocks, a handful of frags, and 3 fish (2 clown 1 royal gramma). I moved to a house recently and decided to upgrade to a 40g. If I had the room or more income, I would've preferred to upgrade higher possibly a 120g, but that'll have to wait.
For my question, I have a GHL profilux mini controller, and a mitras light bar from my 20g that I've been using for my 40g at the moment. I'm sure I need to upgrade my light for proper development of my corals. My biggest issue right now is heating. I've blown through 2 150w aqueon pro heaters and this third one I have it set on 80F because it keeps running colder (my temp probe is reading 73-75F).
I am not the best at this hobby, I've been focused on finishing veterinary school which I now have so I would like to truly spend time getting into and improving this hobby. I know I can set up a heater with my Profilux, but I cannot find a good resource for which kind and I'm going insane. Geometry Dash Lite
My shop I buy from does not use or know GHL well enough, and truth be told, I feel that I may have several issues thar he can't answer himself. New fish die easily, snails, shrimp, they've died. And I'd say 50% of my coral die relatively quick, and my frags I've had for awhile are not growing.
But main focus right now is heating. If there are specific ones you would trust I would love to hear it!