co2 questions

I have a 3'-6.5'-6' room and I am about to start flowering soon with my 400w hps...only two plant but very bushy and quite big... Well I was woundering if i should get a co2 tank for flowering? Also if you think its a good idea for me to get a co2 setup what size tank and how often should I have it running? I hear that 1350 is optimal ppm for co2 in a cann grow is that true?

Try using dry ice.... I use itt when ever i water...