Any time I've medicated a fish I've used a separate hospital tank. I've also done 50% water changes daily, which is really what is needed for HITH. A vitamin enriched diet is essential as well. Since this is something usually seen in larger messier cichlids, soaking a quality cichlid pellet in a little liquid vitamin is usually done.

After looking into this even further, internal protizoans can affect the uptake of nutrients, over time having the same effect as a poor diet. Poor diet & poor water quality are implicated in HITH, poor uptake of nutrients would give the same effect as a poor diet.

In my experiences medicating with metro, 5-7 days is often a little too short a time period, I med 10-14 days, and have yet to see a recurrence with the longer med period. I've had a few recurrences medicating for a shorter period. Yes the fish will seem unhappy by itself, this takes a bit of getting used to by the fish as well as the aquarist. By doing this you are avoiding medicating well fish, can target feed the ill fish much easier, and have an easier time with water changes or any other water parameters that need adjusting.

That injectable is a sulfa based med, odd in that my Dr. prescribed a sulfa based antibiotic for me earlier in the week. Nasty flu-type symptoms, off work for a few days, which is unheard of for me. The meds are working great for me, if they work half as well for your fish it should be well on it's way to recovery.

Thank you Colin & Tolak. I "think" we're on the upfront and it's improving / healing. No more postules and no further evidence of increase in hole sizes. Fish appears well and healthy & alert. It's had it's last injection of Tribrissen and last night I fed the whole tank some pellets soaked in the Flagyl. Not ideal (at all) I know (and understand the consequences of medicating fish where not needed), but the Angelfish just wasn't looking well at all in the hospital tank. Even leaning to one side and flipping over - and partially resting on the bottom - and not eating. Whereas back in the main tank it's zipping all over the place & eating as normal again.

I'm still not sure how it got the HITH in the first place. I certainly don't consider my tank overstocked (tank stock in my profile info), it's extremely well maintained with strict water changes & dechlor. Double filtered and a very varied diet. So just don't know on this one.
But at least I've learnt something about HITH. I'll post back here in a week or so just to (hopefully!) give the final thumbs up.

The bug that causes HITH is in most tanks. It would have come from the LFS originally and chances are any fish you buy will have been in contact with it somewhere along the line. Many fish can live with it indefinitely but when they get old and weak then they become more susceptible to its effects.

Just to say: I think we're cured

excellent, really pleased Bloo

Excellent news Bloo, topics like these are what keeps an old fart like me coming back here.

Thank you! it was with all your help