Several points to attack here, I can’t help with the first as I have no experience with that product.
I like never test my water unless I see a problem
I would start testing. You have corals which require stable parameters (even more so sps). It doesn’t take long and is essential for your coral’s health. You are keeping them alive (just) currently by the sounds of it and we want to promote a thriving environment for our inhabitants.
At minimum I’d test for calcium and alkalinity, but feel free to add magnesium for reference, and phosphates and nitrates.
but when I do a water change the corals go back to looking good a day after once the parameters go back up and I get a bit of growth.
You are lucky, but they will bounce back when the parameters return to what they should be (or closer anyway). Corals like stable parameters and this is arguably more important than a specific target parameter hence why we often talk in ranges for dKH, calcium, etc.
Should I be doing weekly water changes rather than bi weekly?
I am assuming based on the complete post that this ‘bi-weekly’ refers to once per fortnight and not twice per week.
This is probably not frequently enough. Perhaps
switch to doing what you are doing but once per week, or half what you are doing once per week. This will help maintain a better level of stability.
Should I start dosing magnesium?
Possibly. This is a complex topic and you should do a good amount of research before embarking on magnesium dosing as it play a part in the levels of calcium and alkalinity. Dosing mag may help to elevate both calc and alkalinity due to the increased ability to bond, you will do well to read some publications/posts by Randy Holmes Farley.
My ALK stays about 10-12 which is what I want for my corals but I feel like the calcium a magnesium dip as time goes on which is to be expected.
Is there a reason you have alk that high?
No issue there but it is not necessary per se, and if you start dosing calcium your alk will possibly take a hit. Again, research into publications by Randy will be of enormous help in understanding the interconnection between these molecules and proper methods of dosing (where required).
Overall, there is a lot to go through there, but I would advise testing regularly, water changes more frequently to help mitigate swinging parameters especially if you plan on keeping sps thriving.
Hope that helps :)