UPDATE
The Mudskippers have grow slighlty now. One of them (the larger one) is evil, he lives in a cave and he has constructing quite a large burrow out of the soily-mud (I'm pleased I included that now), the burrow is pretty big and he stays in there from time to time. I partly changed the layout so there's another beach at the other end in the hope the other guy might build a burrow.
The less-evil one is very tame now, he clambers onto my hand and I feed him myself. Recently they've been enjoying a diet of mosquito larvae, bloodworm and earthworms. Their colour is pretty good.
I'll try and get some more pics...