Fast growing plant in high ph / high gh / high kh water?

why not keep hydrilla?
If you're keeping cichlids with the plants the plant will be eaten anyways. So, you'd hate to spend money on plants when you can get it for free.

Just make sure to disinfect the plants before introducing it in your tank with a bleach solution or hydrogen peroxide, or potassium permanganate

you can have other hardy fast growers: hornwort, hygrophillia, elodia.

ps. the floating plants are great at absorbing nitrates like duckweed, water lettuce (Pistia stratiotes), and water hyacinths.

For your water parameters, Vallisneria might be a good choice. More generally speaking, there are a lot of fast growers that are nitrate sponges. It depends a bit on the setup of your sump/filter tank.

Wisteria, Water Sprites, Rotala, etc are fast growers.

Pellia has to be the highest biomass producer in my tanks (fairly high GH/kH), might be worth checking out.

Awesome, thanks for the recommendations.

What type(s) would be suited to a moderate flow sump that grew kinda short? I would like to use something like a 20L or a 30L (L meaning Long not liters ) with the water level well enough below the rim to avoid floods during power outages. Sump flow would be 300-600GPH on either a 75 or 125 gallon tank.

Does this sound like a decent/effective way to deal with nitrates? Or will they just not suck up enough? I'd like to keep the main tank so I could vacuum the gravel regularly so no plants in there I don't think The sump would have its own substrate, probably flourite, that would be undisturbed.

FYI my kH can be as high as 27. My gH is very low and I can set it with calcium and magnesium salts. This is for my tap water here, which I hope to be able to use instead of RO/DI.

The floaters sound very nice. Do they root any at all? If not can I ditch the flourite and just do iron additions to the water column along with P and K? Or do these just grow like heck anyway? Sorry for all the questions...

here's somebody who built a planted sump?
http://dataguru.org/misc/aquarium/plantfilter.html

it's not pretty, but it works.

it would be cool if you have the sump outside to take advantage of sunlight and not worry lighting it.