Finally, under gravel filters cannot be used with fish that are serious digger.
Actually they can.
Digging around in the gravel will not dislodge bacteria, just like giving sponges a good squeeze won’t, since the bacteria produce a very strong glue/cement to attach themselves to the surface they’re in contact with.
I’ve known, and kept, even extreme diggers like various large cichlids that will try to hide by piling all the gravel up against the front glass, on many occasions (more commonly the end glass when a tank is end-on). You never get an ammonia reading. UGs even work well when a large majority of the plate is exposed in this way. There is still some flow through the gravel, wherever it is.
When I started keeping fish over 50 years ago UG was all we had. Then ‘power heads’ were invented and reverse flow UG became popular. Then canisters were invented and UGs slowly declined, mainly because they were quieter, less ‘obtrusive’, and easier to clean. Also because they were new. Manufacturers are always inventing new stuff and telling us we need it.
Two pieces of advice from me about UGs…
1. Don’t use a layer of wool between the gravel and the plate. It clogs and slows down the flow, and eventually needs replaced, just like in a canister.
2. Cleaning out the mulm that builds up under the plate is a) not necessary since the mulm is inert and will stay where it is, and b) very easy. Just remove the airline and push a siphon pipe down the uplift right to the bottom. This will pull out all the muck. This works best if the uplifts end below the surface, or if the uplift is in two parts, as in a tall tank, and you can remove the top part, since it will pull water down the other uplift (one at each end?), and you can get all the muck out in one go.