Hi Fat Albert, and welcome to Loaches Online!
I'm sorry to hear of the problem with your
Botia kubotai.
Glad to hear that your water tests are ok, do you have the exact levels to hand? I am wondering whether you might have an elevated nitrate reading if you are only carrying out 15% water changes fortnightly. Your tank seems reasonably heavily stocked, and bigger fish such as the severum and the gourami are likely to produce fairly large amounts of waste.
Botia kubotai are river fish, and as such, require clean well-filtered/oxygenated water - ideally you want to be changing approx 15% of your water a couple of times a week. Even weekly changes will be better than your current regime.
What sort of filtration do you have on the tank at the moment? Is it a Jewel aquarium? If so, do you have an external as well as the internal provided?
It is possible that the smaller of the 3 loaches is being picked on by the other 2. Whilst it is great that you have got a small group of them, some species of Botia can do better in slightly larger groups, say 5 or more (although this is going to be difficult to balance in a tank that is currently quite full). Having more will mean that the aggression is spread between the group, rather than just one fish bearing the brunt of it all the time. Whilst
Botia kubotai could be described as relatively peaceful loaches, it is not uncommon for them to squabble amongst themselves.
As for internal parasites, all the loaches purchased from our shop (Maidenhead Aquatics @ Crowland) are kept off sale and observed for 4 weeks before putting them on sale. During this time, we treat them for internal parasites with Levamisole hydrochloride, to minimise the chances of Chronic Wasting Syndrome aka 'Skinny Disease'. They are also intensively fed to ensure that they are strong and healthy before they leave the shop. So I think it would be safe to say that you probably shouldn't be too worried about internal parasites at this stage.
One other question. What are you currently feeding your fish on? Have you seen this fish eat anything at all over the last few days? You could try feeding after lights out to see if this will encourage him to eat, and observe for a little while to see what happens.
I hope this helps, and sorry about all the questions but it will help us to build up a picture of what is happening.
Emma