I hope you use a brita filter for drinking water, lol. Yah plants will like those nitrates. If I were you I would just go a head and invest in the RO/DI then you can have rams and a saltwater tank.
Here is a good source for DIY co2 it may seem a little complicated, but just use it as a source to answer extra questions that may come up.
DIY CO2 System for Planted Aquarium
Here is a quick and dirty guide from memory, lol
I would start with one bottle (I like 2 liter soda bottles) then you can add more bottles for more co2 if needed. Drill a hole in the cap slightly smaller than air line tubing.
Cut the tubing to a point and pull it through or buy some rigid tubing and silicone it through the hole in the bottle cap.
Then buy some bakers yeast and a bunch of sugar at the grocery store. Mix water (a little above room temp) with the yeast, stir occasionally for 10-30 mins.
Meanwhile add 2 cups of sugar to your 2 liter. Then fill it up with warm water. Let the water cool to around 75 degrees F. Shake. Add the activated yeast. Shake
Screw on the cap and connect the out going airline tube to a power filter in the tank.
This was just a brief intro, but it will work. There are much BETTER methods that you can find just googling around. You don't want to contaminate your brew so there are some tricks you can find like boiling your sugar water first. Just never get your yeast in hot water or they will die. I think 80 degrees F is the max.