For Robert's comments, look at the top of the main GL pages for the thread on Nuggets from a Plant Talk. That is about 2% my info and about 98% Bob's info. That is why it is so useful. ;)

Don't know about your 20-gal and the ammonia. How heavily planted is the water sprite in that tank? If the ammonia cycle and plants can't take care of the ammonia from that (relatively) few fish, you certainly do not want to add ammonia to that tank! Leave the light(s) on a couple hours longer and see if the ammonia is used up.

Most aquarium plants (but not all) will use ammonia before the other nitrogenous wastes. While that may slow down the spread of "good-guy" bacteria, it will stabilize the aquarium.

Glad to hear that you have some water sprite. If the bottom gets a lot of light, in a part of that 10, gently plant a water sprite there. (You know this but...) dig out a hole with your fingers, hold the plant in there and gently roll the gravel around the roots. You will find that the leaves are different. Lights, water depth and all will cause them to vary a lot. (And when it grows out of the water, the leaves look a little bit like carrot leaves.)

A guy visited my fishroom and swore that I had different species of water sprite in adjacent aquariums (five side-by-side 10s under a shop light, on a rack and with windows on two sides). While there are indeed at least two, maybe three species of water sprite, those were all the same species and from the same starting stock. The difference in light and substrate caused the plants in each tank to develop a little differently.

The remaining nitrogenous wastes (because I can't spell nitrites and nitrates) may be used up if you run your lights longer. (I know that is expensive, which is why I switched to mostly 4 foot bulbs in shop lights and am now moving over to the more powerful, but more efficient T-8 fixtures and bulbs.) Maybe 16 hours is about max. There is some resting and some growth and maintenance functions, which most plants do best in 5-6 hours of darkness.

I have a real gripe about some of the commercial aquarium hoods. "They" too often only put 1 15-watt bulb in there. Those 15 watt bulbs often cost as much as a 30 or 40 watt bulb to replace. At the 2 watts per gallon rule, those bulbs usually are really inadequate for aquariums. There are several companies with kits, other hoods and retrofit kits for hoods. Some of them can be found on-line. Go to http://fins.actwin.com/ and search for those topics, if you want specific recommendations from plant aquarium people.