Hi Dude and welcome to MASA
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I also had some problems with GHA and Red slime algae (Still not 100% gone). There are a number of things that you can do to rid your tank of all of these nuisance algae's and get a nice healthy system going with no nuisance algae's.
1st off, you need to use RO water for any water changes, topping up. If you have your own RO unit great, If not then buy it from the LFS. Get them to use a TDS (Total Disolved solids) meter to show you how pure it is. The closer to zero the better. Dont let them tell you that a reading of 50 is fine...its not!!
The guys above said switch to playsand or aragonite (did someone win the lotto?!?!?!)
I recently switched to play sand (R40 for a 20kg bag) and it works great! Just wash it really well before adding it to your tank and do it slowly over a week or two. Slowly remove the course coral and replace with the playsand!
I would get rid of the canister filter as this would be PO4 Factory for your tank. The reason you will be getting a low reading is very possibly because the huge amounts of PO4 is being consumed by the GHA and making it fat and well fed
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Try to get a PO4 remover like Pirugen (I see you have Phosguard) in your canister filter. The sera stuff and all the rest can go. This is just a nitrate factory. Basically all the detritus is just getting trapped in there. Canister filters are really no good in a marine set up!
Do you test your Alkalinity? dKH is very important for maintaining PH and also to low dKH will lead to nuisance algae taking over your tank. Somewhere between 8 and 12 is fine. dKH above 12 with corals is not recomended.
Some photos of your system would be great. Your tank is also gonna go through a mini cycle when you add your DSB with the other sump!