As previously mentioned, DIY CO2 is effective and better than having no CO2. Not sure if it will help with the carpet. I think that is more driven by light. In lieu of a true carpet, I went with Starogynes repens because it can be maintained at a low height. I have it growing in my low light shrimp tank and it is doing okay. Okay as in very slow growth but looking healthy. Another nice plant previously mentioned is Lemon Bacopa.
Crypts come in a wide variety of shapes, sizes and colors; wendtii is one of my favorites.
IMO the other tank without the background looks fine without one. And the one with the background maybe change it to a solid background (sorry mom).
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