How Many

[weejen]

i was just wondering roughly how many plants u can have in a tank before needing co2. my tank is 100x24x15 (LxHxW) with a 39w Hagen Glo T5 HO LINEAR FLUORESCENT light

thanx

 

[RadaR]

It's not necessarily based on how many plants you can have before needing CO2, it's based on light.
More light means the CO2 demand increases and with that, so will nutrients. Lots of light without CO2 and nutrients will gave you big algae headaches because the plants can't grow (lack of CO2 and nutrients is limiting their growth).
39W of t5 over your tank would let you get away without adding CO2.

 

[weejen]

thank you. so am i right in thinking my plants just wont grow as fast?

 

[Jallen]

If you have over 2 watts of light per US gallon of water in your tank then you NEED co2 to supplement the growth given by the light. If you don't provide it then they will go brown and stuff.

If you have less than 2wpg then you don't need CO2 but it helps.

 

[RadaR]

thank you. so am i right in thinking my plants just wont grow as fast?

Yes.
Although I'm sure some of the fast-growing species like Hygrophila should still need trimming every week or two.