What kind of bulb does this ballast require? Alll l know is 250watt and the screw on opening is like2" the one's ive seen on line look small ,also which kind gives more blue light thanks
Your question is a bit confusing.
How do you know it requires a 250W bulb? There are a couple of bulbs that will fit in a Mogul socket like that. Do you have the ballast - what size bulb does it say it runs? It is extremely important that the bulb and the ballast match as running a bulb on the wrong size ballast can result in bulb explosion or burning out (one or the other) prematurely.
MH bulbs come in 10K, 12K (not a lot of these available), 14K, 20K..the higher the K number the more 'blue' you will get out of the bulb. Typically the higher the Kelvin rating, the less growth too.
A 14 K bulb is often nice as you will get good coral growth and not too yellow an appearance. It can also be supplemeted with a couple of actinic T5's or Blue LED lights to increase your viewing pleasure.