While im pondering more plants, can anyone chip in with some ideas? Would really appreicate it! Amazon Swords so far have been complete fail, i'm really looking for a plant that grows big, with large leaves just to fill it out a little.
Those in the photo look good and healthy...how are they failing?
I've grown a couple of species and my experience - with Excel + ferts - is that they took a while to really get going.
I bought one plant of Echinodorus amazonicus back in November last year thinking it would stay quite small (major research fail on my part

) and for a couple of months it did. Then round about January it absolutely took off and the leaves went from being about 5 inches long to over a foot. It practically filled my 64L tank all on its own
How long have you had yours planted, and what species are they? If they haven't been in all that long, then maybe give them a while longer to establish? My theory was that they needed time to get their absolutely
humungous root systems going before the top growth really kicked in...but that could be complete rubbish
As for alternatives, something like Nymphaea rubra maybe? I haven't grown it, but apparently it grows really big, really fast, and as long as you remove the emergent leaves it will stay as a submersed plant. Might be worth a look.