"Butt floating" is a classic symptom of a bloat infection. If the fingerling comes from a hatchery (most likely), I'd surmise the bloat to not be parasitic or protozoan in nature. The more common cause would be a bacterial bloat infection. I've posted the treatment several times lately. But, to save you the search (always a good idea while you're awaiting responses), here's the treatment regimen I offered:
To treat Bloat, the plan of attack is:
Metronidazole and water changes, water changes, water changes!!
1. Remove carbon/resins from filter.
2. Perform a 30-50% water change (reduces free-floating bacteria population)
3. Add salt (teaspoon to a tablespoon per gallon) to aid the fish's osmotic regulation processes.
4. Treat with 250mg/20gallons of Metronidazole (Flagyl)
5. Leave for 3 days and do not feed.
6. After 3 days, perform another large water change.
7. Replace the salt.
8. Add full dosage of Metronidazole.
9. Wait another 3 days. During this phase, the bloat should be decreasing in appearance. In large fish, the water may take on an ammonia odor from the high volume of urea the fish should now be producing due to reducing the bloat and recovering renal activity.
10. 3rd day, another water change. Also, replace the salt and redose the metronidazole (full dose). If the fish looks to be well on the road to recovery you can offer a small feeding each day.
11. Another water change. No meds but, I'd recommend the salt. You should be clear now.
This is the treatment I use on my africans, puffers, and my lungfishes/polypteridae. Knock on wood that it's worked every single time.
Good luck with the sturgeon.
