Lemna minor grows best at a fairly low nutrient level. It does not compete well with algal blooms.

I suggest boiling one wheat grain in a little distilled water and then make up to a litre volume. Put the duckweed on top. They should double every three days at 22ºC. I have read that sycamore / maple leaves are also a good nutrient source.

pH 6-8 is OK, not outside that.

Stephen Tomkins