Why don't you go with an overflow box in aquarium? Especially if you already have fish in it.
I have similar setup, 30g tank and 10 refugium.
First I had DIY PVC
overflow 3/4in pipe. It worked great, never lost syphon, but it was not enough flow. Maybe with 1in pipe or 1.5in would be better. Anyway, I found used 600gallon overflow for $30 and now everything works great.
My fuge has 3 compartments. First, where water comes in, has uv
sterilizer, heater. Second, the biggest, has chaeto and deep sand bed some rock ruble and few mangrove trees. Third, return pump and float valve for top off.
I will take some pictures and post it here later.