Just to add a couple points.
Not sure what system you use but as Zach said the makers are getting better. I use the Canon 30D and Im not afraid to shoot at ISO 800 at all. Can and will shoot at ISO 1250 or 1600 when I really to in order to get the Shutter speeds but Id rather not. Still I have alot of shots taken at iso 1,000 or 1600 where the noise is barely detectable and there are some programs available for reducing noise, so that as long as you dont overdo it, you can improve the image with regards to noise.
Fast Glass.
Nothing replaces fast glass. Get yourself a nice 50 f1.4 or 85 f1.8 or some other fast peice of glass. They are great for portraits of people and fish due to their narrow DoFs
f1.8

And when you need to you can open them up for low light shooting.

I also use the 85mm f1.8 for my kids indoor Volleyball and gymnastics where even f2.8 isnt fast enough at iso 1600 to freeze the action and it works well.
Which ever system you use you should be able to find a 50mm 1.4 or a 85mm f.8 or something very similiar that wont break the bank. The key get atleast one peice of fast glass, you’ll quickly love it as a portrait lens and really appreciate its ability to gather light in low light conditions.
Lastly
Shoot RAW.
Setting WB in those conditions is tough. With raw you just let the camera do it, and then correct it yourself in the PP phase.