Water Changes

The problem is that in my husband's mind, it's downgrade not upgrade. He thinks a 125, a 50 and a 25 are far too many tanks. If he had his way, I'd have maybe just the 25 litre with a betta, nothing else 

That's something that you'll have to work out yourself for your situation. I hardly ever squeeze out the sponges in either of my tanks, but I keep a close eye on the water flow and if it seems to be reducing then I'll give the sponge a squeeze. It's about once every four months or so for me. Other's may differ.

I have a mini spraybar on my filters and it's easy to see - the jets of water from each hole don't reach as far as they used to. What filter do you have? I presume the outflow ripples the surface?

I've not had that particular filter, and even with filters that have a venturi I'm not allowed to use them (husband can't stand the bubbly noise  ) The filter should be positioned so the top outlet ripples the surface. If you look down on the surface you can see how far those ripples travel away from the filter. When they don't travel nearly as far as they used to, check the filter. If the sponges are covered in brown goo, that's the time to wash them. In old tank water that you remove during a water change, of course.