Synodontis eupterus is the scientific name for the Featherfin Catfish, it's also called the Featherfin Squeeker along with several other varients on the names Featherfin, Squeeker, Synodontis and Catfish. I have a featherfin too and I think that's what you've got in the photo - but there are so many catfish I can't be 100%.
1. Yes, sanded, cleaned, primed just the usual really. Seems obvious but certain people I know have just opened the tin and splashed paint on greasy wooden kitchen doors and then been surprised as it flakes off all over the place for the following months!
2. Cured is fully dry yes. Paint is often touch dry in a few minutes or hours but that's just the surface. Enough to put a second coat on perhaps but not dry right through so it can be easily damaged and is still changing chemically. Curing takes from 24 hours upwards depending on the paint type and atmospheric conditions. Personally I'd leave it about a week.
Polyurethane varnish should be completely inert once cured. Prep the wood well then paint and varnish and everything should be fine. Although prolonged direct contact with the water should be avoided the odd drip from condensation will be ok.
Previously I have coated the inside of a DIY wood with tin foil wallpapered on with PVA glue. This held up well and acts as a reflective surface for lighting.
Nudibranch are a type of sea slug and are common in the sea and occasionally hitch hike in live rock or on corals. There are different types and while some are pest control or vegetarian algae eaters many others like corals, sponges and even other Nudibranch. Fascinating creatures.
Tap water contains dissolved gasses and temporary buffers to ensure that it's close to a nice tasty PH 7 for us humans to drink when we put it into a glass. If you leave the water standing these temporary buffers degrade, disperse or otherwise leave the water and the ph returns to its more natural and stable PH.
This could mean the PH goes up or down, it depends on the chemical composition of the water.
More to the point, don't kill the fish. If you can post a photo and details of the tank and water parameters we might be able to suggest a course of action. If you feel that is difficult or would not be fast enough then give your local vets a call and talk to them. Most decent vets have some experience of goldfish, if you're worried about the cost of a vets help mention it to them and they will often try to help you.
Looks a lot like what I built for the rabbits Converted the old playhouse. Now the rabbits are gone the wife wants it extended for chickens next year.
A few things I learned - MDF is no good outside, but exterior ply (sometimes called marine ply) is great.
To stop the foxes fix strong mesh to the base of the coop and extend it 12" into the grass as a border all the way round. Peg it down to keep it in place until the grass roots through it. When the fox digs it digs at the base of the wall it wants to go under - it can't dig through the mesh so as long as the mesh is firmly fixed to the coop you're safe.
Skimmers do need to some time to settle I had the V2Skim-800 model TMC skimmer but in that there was a control for the quantity of water leaving the skimmer. The more the flow was restricted the higher the water level in the skimmer and the less bubbles pulled in by the venturi (and hence less bubbles leaving too. Have you tried adjusting this knob?