Hi..

[Riccia]

Sorry, moss no grass.

Riccia is available as floating (surface) or sinking
(bottom) moss. They need different kind of lightening.

And you also could have fine planted tanks without Riccia..!

Maybe
it's even possible to retrofit a double T5 kit (although
this would bring each
lamp up to 24W - I think!)?

Maybe, but does it make any sense?

But at 24" Arcadia only do 2 T8's at a total
of 30W,

These T8/30W tubes are 89,X cm long and generally used in
100cm hoods.

or 2 (or 4) T5's which only have marine lamps in them.

T5 should also be available for freshwater.

Are marine lamps any good for plants?

Not really.

Then I looked at trying to make my own hood with, say 3
T8's, but don't really have the skills for that.

Should also be available custom made, too. Start an inquiry.

HTH.
--
cu
Marco