This does not appear to be swim bladder related. Tail standing can be a result of some deworming meds, high bioload, nitrates, etc... Cross contamination should produce different symptoms, although I wouldn't rule it out since you do have some new fish.
Increasing your water changes is an excellent start. Once per week water changes can create swings in ph and bioload, so maybe your new schedule will help. I would try to get those nitrates down to 5. Some intestinal blockage could be present. This usually makes them head stand, but I've seen it cause tail standing in the past. 1 TBS epsom salts per 10g for a couple of days won't hurt a thing. As long as no infection is present, turning the heat up to 90 can help speed up metabolism and feeding.
Another option is to place it in a hospital tank and treat with clean water so you can give it more personal attention. I don't know what's in it so I don't recommend this very often, but the makers of Bactocide 2 claim to have great success with issues like this.
This isn't a very common issue, but I'll ask around for some more specific recommendations and check back...