1.)I want to know what are some ways that I can put co2 in a 120gallon tank. I thought about using tablets, seachem flourish, a diffuser, reactor etc.... I am not sure which one to buy. Can I use a combination of co2 methods or what?

2.) Also do I need to replace the substrate in my new aquarium, if not how long does it last?

3.) And since I have a freshwater tank I thought about using a wave maker for current to flow through out my tank but I heard it is used for saltwater tanks. Our there any benefits of using a wave maker in a freshwater tank?

4.) If I am going to add discus what are some other things that I can do to make the water quality for both fish and plants?

Thanks