I agree with what's been suggested. Mikrogeophagus ramirezi
can be quite feisty, but in spite of that they are basically shy fish. When your plants fill in more the existing ram will feel more comfortable. But having more, a lot more, peaceful fish above him (rams remain close to the substrate) will help. And cardinals should be in a larger group for their own good. In that tank, I would add another 7-9 cardinals to your 4.
And a female ram would be good. They form pairs, and sometimes the male may reject the female, but you can only try. In that size tank with hiding spots (more plants, even more wood) they should be OK.
And remove the Puffer as mentioned previously by others.
Some peaceful catfish would be nice, but consider first that if you want success at spawning the rams, forget any catfish. All catfish are nocturnal to some degree, the rams are not, and in a community tank the eggs or the fry will always get eaten by catfish--later if not sooner. This applies to corys, pleco, etc. Farlowella seem to be OK in my experience, but they are not strictly speaking substrate catfish.