Play with that for a while and go from there to improve it.
If you are stuck on wanting both filter and tank always the same water level. Than you rather skip the sump idea go with Hamburg mattenfilter concept.
Perhaps your overflow has slots and is not actually an overflow?
Or perhaps it is well below the waterline and not actually working as an overflow?
You can’t deny the laws of physics of defy science so if your water levers are remaining the same there must be a reason.
Maybe some photos would help explain to us what is happening.
I will have to empty the system and try just filling up the filtration section to disprove this theory but I really don't think this is the case.If both tank and filter compartment stay the same water level i can think off only one reason. Both compartments are not properly sealed from each other and connected somewhere.
So I've been confusing myself for months now, trying to get myself around something I already understood?a ha ... well In that case the filter is in fact joined to the tank until the water level dropped below the slots and in fact you do not actually have an overflow .
It looks like you can afford around 50mm drop in water level before the baffles start to actually overflow.
does this still stand in your eyes?If both tank and filter compartment stay the same water level i can think off only one reason. Both compartments are not properly sealed from each other and connected somewhere.
I'd tried to give the best descriptor I could which is where 'aquarium with filtration compartment' came into play.There are two key words ... overflow and sump... neither of those apply as you dont have either in your system
I knew it was going to be tight and reducing it further was an afterthought.The key thing here is that you've actually tested it too many people don't.
It's got a run dry cut out built in so that's sorted.You might want to think about putting the return pump on a float switch, in the event that there's not enough water in the return section to prevent it getting burnt out.
did you have problems with water only evaporating from the filtration section?
Is it really that abnormal? I know it is a method used in other countries and even widespread with off the shelf products in the UK like the Fluval Flex.Andrew, I think you will be fine with your set up, it is not a normal configuration but I see no reason why it will not work very well for you as long as you keep an eye on it.
Have you considered feeding the pump with C02?