Adding substrate to established tank

[t-ro]

As the title says, I want to add new plant friendly substrate to a 90 gallon tank that has already been up and running for more than a year now. I have regular aquarium gravel in the tank already, but it's only about an inch or less deep. What would be a good substrate I can add that won't harm the fish that are already in there? (I'm not too worried about cloudiness, as long as it's safe) I'm also not sure if I should completely replace the gravel, put new stuff over it, or just mix in a new substrate. I'm just looking to do a low tech right now. I have experience with reef tanks and fish only fresh water, but I've never done a planted aquarium before and want to start out slow.

[Sugardaddy1979]

Is the tank already planted? What is the existing stocking level?