Betta Fish: UPDATE! (Help? Betta in tank for 3 days tank smells like ammonia)

Thanks to everyone for the advice, our little bro is well on his way to a cycled tank. Parameters are stabilizing, water changes are down. Finally seeing some Nitrates. Dude seems happy.

One remaining question: I tried to add some plants on the side with plant clips, using Root Riot Cubes. They were in the water for 5 min before my guy started "flashing" so I removed them and he stopped within 20 min or so. Are Root Riot Cubes not aquarium safe??

Thanks again r/bettafish