Elatine Hydropiper- it's my favorite carpetting plant and I've never even used it.
I've found it's hard to scape with Downoi, for me atleast; I always want to do minimalist things and it doesn't always fit in. But it's a gorgeous plant on its own.
I also really like elocharis belem, it's really nice for a carpet.
Ferts: fertilizers. Like you'd use with a garden, because plants need nutrients. Plants need Nitrogen, Potassium and Phosphate in abundance and other minerals like iron and phospherous to grow.