You want the ppm of your CO2 to typically match the ppm of your nutrient level. So at the height of your plants flowering (4 weeks in or so), when your PPM is around 1500-1600 or whatever, then you want your CO2 to match.

Remember with CO2 that you cant just leak it into the room. You need to cut the exhaust off, let temps rise (90 degrees!!!), and let the CO2 soak for around 45 mins-60 minutes (some people do more). Then exhaust for a period of time, then repeat over and over all day. If you are having a heat problem, this is something to think about and the reason why people use sealed aircooled lights. Also remember that you wont want to start CO2 until buds form and youll want to cut it out 2 weeks before your harvest or the CO2 will COMPLETELY rob your buds of taste and smell. Its horrible!

I just wanted to mention something about your CO2 pricing too. Im assuming your in the area of your name...so you have easy access to Craigslist.org You can find an insane amount of equipment (sometimes for insane prices) for sale. In fact about 75% of my equipment has been obtained from people like you and me. Out of the 30 people Ive bought stuff from, every single one has been incredibly cool and very chill (a key factor in our business). To give you an idea, I got my CO2 tank for $60 and my regulator for $50, and the other lines, etc for around $40. $150 for CO2 (and it even came half filled!). Ive seen PPM meters go for under $200 and Ive even seen $1300 plant controllers for go less then $500!

Remember too, your room must be completely sealed for CO2 (especially near the floor as CO2 is heavier then air). 1800ppm of CO2 is not very good for you.

Oh and remember, growth that has been damaged will not repair itself. As soon as you see nice new green growth, then your plant is ready. Start light and build your way up. In my experience its far better to under fert your plant then to over fert it and lock the stuff out.

PS. I would change over to HPS as soon as possible. MH is horrible for budding. You can always change one bulb first if your worried about switching over. But your plants look very established, I wouldn't worry about stressing them much.