fish swimming nose up/tail down

Question
Rachel, I have a 40 gal tank, set up for about 12 months with an Aqua Clear
70 gal power filter which I set up about 2months ago. During the time that I
changed from my BioWheel filter I kept the wheel itself with the bacteria
colonies on it in the water for the first month. Anyway, I have had all of my
fish for 12-18 months now, 3 cories, 6 flame tetras, 3 lemon tetras, 1 otto
and 6 rasboras.  The otto was the only survivor from the 4 that I initially
bought about 6 months ago, the other 3 died within days- 2 weeks  of
purchase.  I normally do a partial water change biweekly with a more
thorough change ( about 20 with gravel cleaning) monthly. One of my
rasbora( a larger one) started looking a little darker in color about 2 weeks
ago but it acted fine and was eating well. I checked my water quality, ph 7,
neg nitrites, neg amonia but had a 40 on nitrates. I completed a water change
along with filter cleaning/ gravel cleaning last week. My filter did not appear
overly dirty at the time. Today I noticed the one fish swimming with difficulty,
she appears to be having a hard time with keeping her tail in line, tail is
sinking while she is swimming. Her color looks much darker, she attempted
to eat but is having difficulty staying at the surface to eat. I'm not sure what
to do at this time. The other fish all appear to be fine including the otto and I
know that they can be fussy about water quality.  At this time I have placed
her in a mesh basket within the tank to help her reach the food.  She is sitting
around on the bottom of the basket quite a bit.  Help!  Thank you. Karen

Answer
Usually swimming difficulties are due to bladder problems or bloat commonly associated with parsites and diseases. Are there any other symptoms? If not, I would get an internal parasite/disease treatment and treat her. I would suggest treating her in a different tank but if this is not possible you could risk the stress to your healthy fish. Follow the directions including dose and water change requirements. Also, closely manage your ammonia (nitrates and nitrites) as this could also be a problem. Let me know if you need more help.