As for the nitrogen cycle in aquariums, nitrate is nitrate is nitrate, it doesn't matter what it originally came from, as an ion, the makeup is the same. Aquarium plants will most certainly use it, but usually don't have to, as they will also uptake ammonia as it's put off from the fish and it never has to go through the process of being converted from ammonia to nitrite to nitrate as an end product. High nitrates in tanks cause algae problems because there is either not enough plants to uptake the nutrient levels, a lack of other essential nutrients for the photosynthesis process, or some other contributing factor. A heavily planted tank will rarely have nuisance algae problems from any algaes but the ones not easily cleaned from leaves such as hair algae and the dark green calceous spot algae.
Ammonia is actually changed to ammonium in water that has a pH under 6, and is not toxic to fish as such. That is the process that amquel and other "ammonia detoxifying agents" utilize. This can and will cause problems with your biological filter if your pH level falls, and then a water change bumps the pH back up into the alkaline range.
Water changes are the best method of stabilizing your water parameters there are. If your tank has gone for an extended period of time without them, the best method to correct the problem is to do small volume changes more frequently, until your tank parameters more closely match the tap parameters. This will slowly replace the buffering capacity for the water, without yanking the pH around as the built up acids defeat the alkalinity after each change, causing the pH to jump.
I personally use nitrate levels as an indicator for dissolved organics in the tank. I'm not so worried about the nitrate levels, because those aren't hard to manage, but the other trace elements and minerals that can build up over time if they aren't being partially diluted on a regular basis.
Then again, I use the same method of maintenance on saltwater tanks, and while it's not the popular choice, it keeps the tanks healthy and stable, without dosing chemicals that you aren't testing for and risking causing more problems. If corals aren't actively growing great guns, they aren't using more calcium than already exists in the mix. Most people just don't like the idea of having to mix all that salt and what not
If I confused the issue more than I clarified anything, my apologies.
Barbie
