Gravity feed from internal overflow
down to bottom side of wet/dry - flows up thru mechanical filter
spills over into drip tray and goes bio-bale media.
Below bio-bale in the sump area, there is an inlet and outlet to the Fluval
Before water returns from Fluval back to sump area it goes thru UV light
2nd pipe coming down next to return pump is new water feed from
It's all gravity fed from the overflow. Return pump stops and whole
system will stop, no chance of runaway siphons.
The design: the biggest is the high loss of CO2 since the wet/dry section is exposed to the air. If it had a sealable lid, then you'd greatly improve the CO2 situation. You might come up with a good solution there.
eg: Some sealing insulation tape and a glass lid with a latch etc.
Good prefilter design, no overflow, clean that sponge weekly or every 2 weeks and it should really keep things clean. If you added a needle wheel, say a Rio 1000 with a snipped impeller that fed into the suction side of the return pump, then you'd get really good CO2.
I've never liked fluvals
Rena, Ehiem etc.
Your system has a lot of good aspects for automated water changing. I'd try and hard plumb those into the system if you own the home etc.
Then turn a valve to drain(water go to the landscaping), turn another to refill,(from aging tank or tap).
Makes the the water change easy.
Nice skills putting it together.
A lot of thought went into it.