Adding a macroalgae tank as a second sump?

[GothFishKeeper]

I’m very much running out of room for all the filtration i wanna put on my tank. My sump is a 20g long diy build with the fiji cube baffle kit. So I’m thinking instead of using the refugium section in that sump as a refugium, I’ll just hook up all my external reactors and their pumps to that section. I just have cycled rock in there at the moment, no chaeto. I have an empty 20g long from when my ball python was a baby, he’s now two years old and in a 4’x2’x2’. I want to make that into a refugium that also serves as a really pretty night light (i plan on running my lights on a reverse 12 hour cycle). I would love to make it a whole macroalgae tank that’s fully scaped out. Even though it would still be underneath the stand, i could just take one of the doors off. I have a 65g display tank but it’s on a 72"x18" stand. My AIO nano reef is on the other side of the stand. So my question is, if I wanna scape this second sump with sand instead of just rock and limited species of macroalgae, would i need to cycle it completely before connecting it to my existing system? Or can i just add fresh sand and the rock from my current sump and connect it right away? I would put a baffle between where the aquascape and the pumps are of course. I have about 100lbs of rock in my display tank already, but would adding additional water throw off the balance and potentially cause an ammonia spike? I have some relatively sensitive fish in the tank so i dont wanna do it if it could hurt them.

 

[Subsea]

Adding new mixed water to system volume will not change anything.

However, be aware that when power fails or pump fails, system levels will drain to lowest tank. Make sure your syphon break works correctly on drain from tank above.