I actually use them in my sump, and they are very good at reducing Nitrates. If you prune them they will grow more bushy and less tall, so it is controllable with very little effort. As for them being expensive, that all depends on how you buy them. I bought mine on eBay as pods
with root stems and paid $10 for 10 plants. I soaked them in fresh water to start and acclimated them over a week before I planted them in my fuge. Since then they have grown well and are eliminating my nitrates. I do nto see any problem with putting in the main tank except that they do need to grow out of the water and it may take forever if you tank is deep, and then they need some space vertically as well. If you can try them, they seem to work great for me.