I do the same as you - they get fresh water in trugs every morning but it's quite muddy soon after. My understanding was that that would be OK - certainly the breeders I bought from reckoned changing the water daily should be sufficient to prevent eye problems. It's more about making sure it's deep enough, I think. I've bought a big dog bath which is deep enough for them to paddle about in and then I know for sure they can preen and clean all over. Problem is it's so big, you can't change the water daily - just turns the whole run into a mud bath - so they still need clean water elsewhere. OK, realise that was a fairly pointless reply!
H