What if i reduce the light, the plant will grow slower right ?
One would think so!?
🙂 But it's hard to say... Without a PAR meter all you can say you have light. All plants have regarding their genetic preference a different Light Compensation Point and accordingly more or less CO² preferences. And this divides the plants into 3 groups, easy, medium and advanced to grow. Obviously, the easier they are to grow the less light and less CO² they will require. Your weakest link is the most advanced plant.
Anyway since not all specific data is available about all plants we could grow it again comes down to trial and error.
But you could also tune down the CO² contents... In general, people aim for 30ppm since this is the maximum to add and is still relatively safe for the fish. Going higher isn't bad for the plants but the fish will suffer. But it also means you can go a lot lower than 30ppm and it still is beneficial for the plants. There are people that reported adding only 15ppm of CO² and didn't experience the plants growing slower or less healthy. But this again highly depends on which types of plants were grown I guess.
As said before trial and error, there are many ways that lead to Rome. Just try things out and see where it leads to. And there is very little written in stone in our hobby... We have general guidelines to optimize it based upon average data collected and gathered by experiences. It doesn't say deviating from it is a guarantee for failure.
For example, a general rule for CO² tanks is a sufficient turnover for CO² distribution, recommended 10x the tanks volume and you can't go wrong.
I kept a CO² tank for years with a 4 x turnover and didn't really notice negative effects... On the contrary, some plants such as a water lily and the fish I kept in it liked it better.
🙂
And no 2 tanks are the same.