Milky Water.

Milky water is usually a bacterial bloom. Have you done a water change in the previous 24 hours? That can sometimes trigger one in an established tank. And stirred up the substrate more than usual during a water change causing fine particles to be disturbed?
Used any different food?
Used anything different eg new dechlorinator?
Having said that i did read once that algae blooms aren't always green, they can be white too.

As long as your ammonia and nitrite stay at zero, I wouldn't worry too much. If it gets worse we'll have to decide what the cause actually is.

Whatever the cause of the milkiness it could be affecting the oxygen level of the water. If it is a bacterial bloom, the bacteria use oxygen like the fish do. That's the main way bacterial blooms harm fish, by using some of the oxygen. The fact that your fish are hanging out nearer the surface (where there is more oxygen) or darting to the surface more often does suggest depletion of oxygen. Ways to get more oxygen in include using an airstone if you have one (churns the water up more efficiently allowing greater gas exchange) or by turning the filter flow up/pointing the outlet more towards the surface.
The white could also be 'stuff' loosened in the sponges and the flow carrying them into the tank. How heavily did you clean the sponges? A light squeeze or scrubbing them heavily?

Re the threadfin - that's why I always cover fry cages. I use a piece of plastic craft mesh which allows air in but not fish out.

The sponges and the fine wool was really clogged with brown algae so i gave them a good rub,maybe I over did it    I always use PURE in my filters as well, it's great stuff Cod.

Switched the filter to max aeration and turned up the air pump to max. I should have thought of that!! Thanks Sue. The fish won't like the current but it's better than suffocating I guess.

I found the threadfin and it was dead sadly so I hooked it out. I'll double check for bodies before I go to bed.

Mulling over adding some EshA 2000 in case it is bacterial but I doubt the fish need anymore stressing. Anything else I can do that I've missed?

I've got a feeling Sue's right as usual and I was a bit too vigorous rinsing the filter sponges, kind of odd though as I have ceramic media in there as well and the tanks been set up since xmas last year.

The water is crystal clear again but I've been doing 12 hrly tests and they've picked up traces of nitrites so it seems I have a mini cycle  on the go, what a pain!

I try and avoid disturbing the substrate too much but maybe I did, although thinking about it I have a nerite that spends its time under the substrate, popping up to the surface occasionally, so maybe that hit a small gas pocket.

Apart from the poor threadfin which was one of my oldest, no more deaths thanks to Sue's prompt advice. So thankee  again 

I love this forum!

edited to add, head count when the lights came on has me down 2 CPDs, no bodies though so hopefully they're hiding, it's either that or the other fish and snails clened up for me