Hey, how long have you had the trig for? Is it resting on its side or trying to maybe scratch the area where the bumps are? How is the breathing? Is it rapid? How is its eating? Does it still have an appetite? Any other fish in the tank? That way we can rule out certain parasites which can only spread from interaction. Where abouts on the triggers anatomy do they appear and are they in more than one place? If so, where did they first appear?

If you check back before anyone replies to your pics you could always have a look on this website

Hi,
Do you know how many litres (on average) your display tank drains to the sump per hour? What is the height from the place where the return pump would be to the re-entry to display tank? Height can compensate the pumps turnover rate due to vertical travelling so a pump with say 1500 l/ph might only be 1250 l/ph or less. Good rule of thumb is to match, if possible with the return pump abit less of a turnover than the display tanks draining. This has worked for me and in the tanks I have setup @ SeaLife.
As for bulkhead, 42mm are hard to find for general use because they are only used on bigger tanks, the most common 'big' bulkhead in most LFS' are 40mm but you could find some online or perhaps try a reducer as last resort?

that's just Jim the worm

I second the ammonia explanation.
The size of the tank may be okay to keep juveniles of the species but should only have ever been a short-term plan when keeping tangs in an inter-species environment. Larger tanks absorb changes slowly and thus you would have had more time to treat the problem, which become increasingly concentrated with decreasing tank size - this would explain the sudden deaths, particularly the species which have died - the tang species and especially the dwarf angel are incredibly intolerant of chemical changes, clowns - being damselfish, are somewhat hardier.
As already suggested, a water change is the best option right now, and keep ontop of the water parameters. I'm sorry for your loss, its not a nice thing to have to deal with.
If the tank size is limited by your houses roomplan in respect to the dimensions it will allow, a thing to consider when keeping tangs is that they like to swim and glide through water in lengths, so a long tank is more preferable than a tall or deep one. If you decide to restock with tangs.

Which species? Small as in less than 10cm length? 1 frozen cube on mysis/brineshrimp every other day should suffice. Try to mix it up a bit though.

What colour? Emerald crabs and teddy bear crabs are common hitchhikers, both are harmless unless you keep gobies or soft corals. If its an emerald crab then consider yourself lucky, they retail for upto £25 in some places. Try taking a pic.

If you want to keep it in check, for instance; if you have inverts with specific needs. Then here are some "usefuls":

Red Sea (the saltiest sea): 1.027 - 1.030
Indo-pacific: 1.021 - 1.025
Carib-floridian region: 1.023 - 1.026

A general rule of thumb is a range of 1.023 to 1.025, although inverts are more sensitive to salinity than fish the recommended range also takes into account the efficiency at which nitrifying bacteria work. They run more smoothly at lower levels of salinity because more oxygen is kept in the water.

Shouldn't be problematic.

Assuming that your refractometer is calibrated at 60 d F (which is the norm) distilled water equals 1.000.

Two things to consider:

1. Density
2. Temperature

When you took the s.g. as 1.027 what temperature was your tank? Assuming a range of 77 d F to 84 d F your salinity could be anything from 37.8 to 39.4 ppt.
In any case no need for alarm.

If you want to lower it just scoop out a few cupfulls until the s.g. lowers and then top up with freshwater.

I'm moving house in Feb and will not have any room for my current setup - A Juwel vision 450 with lots of extras. I have listed it on eBay. Not sure if I'm allowed to post a link but if anyone is interested in a more or less complete marine setup please get in touch and I'll pm the eBay link.