I had a small brownish outbreak, cloudy and yellow water during setting up so I got the smallest uv lights I could find, one of the green killing machines. There’s probably better product out there but this one worked for me. I’ll probably just keep it running.
When my tank was new (still is) I had a dino outbreak. My tank cycled for like 2 months and then after about a week after putting stuff in it and new lights I had a dino outbreak. I got the Green Killing Machine 9 watt (because it's cheap) and it completely cleared it up in like 2-3 days. I read many things of people saying it wouldn't be strong enough, but I figured what the worst that would happen? If it doesn't work in like a week or two I can just return it. Needless to say IT WORKED.
I'm fond of the Aqua UV Advantage series, little HOB UV systems. They're not submersible, but I have one on my 20g IM peninsula and it's worked great. I personally wouldn't recommend the IM UV sterilizer - I had one on another tank, but the pump included was somewhat shoddy and the cord ended up tearing its insulation. Didn't realize how bad it was until I saw the smoke coming out of my sump cabinet. Fortunately I was there when it happened, otherwise it could have been really bad.