Ok. I have some seachem alkaline buffer.
Replacing the water with rodi is a good thing isnt it?
IMO, not always. There's a lot of stuff you need to put back in the water to make it suitable for fish.
That said, using RODI is a good thing if you are certain you can end up with better water when you are done because your tap contains a large amount of something problematic.
Others may differ in this opinion, but many seem to be under the impression their tap water contains horrible things that their fertilizers, and GH/KH boosters do not. Many of these are, IMO, expecting their fertilizers and GH/KH boosters are free of contaminants out to the ppb level. Sorry, you are almost certainly adding the same trivial traces of lead, arsenic, pthalates, and other things to your tank in this process.
The main reason for wanting to go to rodi is so i can decrease the ph abd hopefully increase kh and gh. Plus the other benefits of reduced algae, reduced effect to changing ph in tap water and other things that might be in the tap water.
You can't really decrease pH and increase KH. Any KH booster you add will raise both KH and PH. The only way to decrease pH without affecting KH is to inject CO2.
But why are you trying to lower your pH and raise KH anyway?
Plus the other benefits of reduced algae
How does RODI reduce algae?
If your tank is planted, you have to add back all the things that are necessary for plants to grow: nitrate, phosphate, potassium, secondary and micronutrients, or your plants will die too. Algae is a plant, so once you've added this stuff back, it has all it needs. Trying to suppress algae through nutrient deprivation in a planted tank is an effort in plant suppression too.
Non-planted folks may seem some benefit from the reduced phosphates, although there are other ways to do this. Things like phosgard will also take out the phosphate added by your fish food.