I am fairly new to reefkeeping. I am currently running a 10gal nano FOWLR howeverI plan on running a 250W MH fixture on my new tank. The color temp of the bulb is 15.000K. Do I need actinic supplementation for when/if I start adding corals? Or will the 15,000K be fine?

Many of us run actinics with MHs to bring out the vibrant colors in corals. What size tank is your new one going to be?

I hope you aren't going to put that 250w over a 10g! The water would be boiling in no time. :D

Its going over a 35gal hex.

That would be overkill. Too much light, too much heat.

are you talking about for the 10g or the 35gal?

35 gal. Why do you want so much light?

The tank is 24" deep. Theres only one bulb it's not a multiple bulb setup.

Even at 24" deep thats way too much light, especially since everything will be directly under the light. I would do 150-175w pendant over a tank that size. Plenty of light to keep anything.

i would go with the 175w MH. 15k, tells me it is an XM bulb, and the par really stinks on those bulbs. i would recommend a 12k Reeflux over that 15k XM.

okay. I went with the 250watt because that is what about every site i coul find on lighting said would be needed for that depth. Could I run a 175watt on a 250watt ballast?

nope, the 250 ballast would burn the 175w bulb up in no time.

Thats what I thought.

Does it make a difference if its a HQI dubble ended?

yeah, it probably wont even fire a single ended bulb. but since you already have the ballast, try to find a pendant without the ballast for a double ended bulb that has a couple of actinic power compacts in it. i think that 250w might be a little much for the tank size, but we should be able to work with it.

Thats basically what this fixture is, I think.