My experience with denitrators
I have three large planted tanks and have been using denitrators for more than a year. I just wanted to give you some idea about these devices.
There are two basic kinds that I know about. I won't mention brands but simply types.
1) Those that need to be fed with a carbon source (usually either ethanol or sugar water)
2) Those that are fed by sulphur balls that are inside the denitrator.
I have used both. Both take just a bit of messing with. Neither kind are maintenance free.
The following are some random observations.
1) Both of these kinds of devices can be potent fish killers when they are being broken in. The reaction in both cases is
(a) ammonia to nitrite (in the presence of oxygen)
(b) nitrite to nitrate (in the presence of oxygen)
(c) nitrate to nitrite (anaerobic)
(d) nitrite to nitrogen gas (anaerobic)
Each of these steps is done by a different kind of bacteria and each kind of bacteria can't start building up populations until the step before it is complete. There are two places where nitrite is produced and nitrite is way more deadly than ammonia (especially if, like me, your tanks tend to be on the acid side).
2) For both kinds of devices you should let the denitrator be your main (only if you can fix it that way) nitrogen cycle device. You need the oxygen using stages to occur in the denitrator so that the oxygen will be used up, allowing the anaerobic bacteria to thrive in the second half of the unit.
One thing this means is that your tank is very dependent upon the denitrator and if something happens to it (it gets stopped up, etc) you suddenly don't have any nitrogen cycle going and things can go bad (ammonia spike) in a hurry.
Another thing it means is that all you really want from your canister or other filter is to remove turbidity from the water. Filter maintenance becomes really easy and filter floss or sponges are all you need. (That said, I always have a large canister for turbidity control and a separate loop for chemical filtration, discussed below).
3) Every time I tried to feed one of the carbon-eating denitrators with ethanol (cheap vodka as recommended by the manufacturer) I ended up killing fish. Maybe the food was too good and drove some bacteria population to move too fast, possibly leaving me with a nitrite spike. Every time this happened, by the time I saw it the tank water was too messed up by massive numbers of dead fish for me to figure out what the culprit was.
I therefore feed these units with simple syrup. Household white suger mixed with an equal amount of water and kept in a jar with a tight lid. The jar gets a little alcohol in it if you keep it for a while, as I often do. You can smell it in there. This has never seemed to harm the fish.
4) Once the carbon-eating denitrators become fully mature they don't need to be fed very often and you can run them as a steady stream instead of a drip. This reduces maintenance by a lot.
If you do feed them too much they will eat up all the nitrates and then turn to eating sulphates. When they do this they release not nitrogen gas but hydrogen sulphide. Think rotten egg smell. So you have to be very careful not to overfeed. Or keep the windows open.
5) With a mature carbon-eating denitrator it is very easy to keep ammonia, nitrite, and nitrates at 0, 0, 0. You don't need to do water changes anymore unless you are putting in too much fertilizer. You are better off with a system in which you measure nutrients and just put in what you need rather than one where you put a certain amount of stuff in every day or week whether you know you need it or not.
6) Once you stop doing water changes both sulphate levels and phosphate levels will start to rise. Sulphate levels don't do any harm that I know of. Phosphate levels are probably bad for causing algae outbreaks. You can reduce phosphates with GFO (granular ferric oxide, a fancy way of saying rust) in a reactor. This can be a high maintenance operation because the GFO changes to something else when it reacts with phosphate and that something else tends to clump so you have to keep changing the media a lot. The stuff is expensive. Be careful not to put in too much and waste it. To me, GFO is way better than any other commercial product for this purpose. Some of the commercial products seem to have no effect whatsoever. Get yourself a Hagen phosphate checker. Great device.
If you have fish in the tank you will never need to add phosphates for fertilizer, I promise you that.
I use RO/DI water with a TDS of 0 (or 5-10 if I haven't kept my RO rig maintained very well). This is an absolute requirement for this kind of thing because topping off with water that has even a little minerals in it will make the mineral content of the water go off the scale over time. At the same time, plants use minerals, so you need to measure things like calcium/magnesium and make sure there is enough in the water. Most plants, even plants from soft water areas of the world, actually do better with hard water.
Also, make sure you keep the carbonate hardness at a decent level. Otherwise you could have an acid spike. This is another way to kill lots of fish.
7) So far as I can tell, the fact that the nitrates are always zero does not seem to hurt the plants at all. It doesn't hurt the algae at all either. I have a consistent problem with black beard algae, and I have seen some of the dreaded blue-green algae. I have good luck with the UltraLife blue green algae solution. It will ultimately come back but it can kill it for months at a time. I have no idea how to deal with black beard algae and I have just learned to live with it. If the plants are growing fast enough and you can trim out all the old leaves it won't be too bad. In my tanks it tends to cover rocks and wood like a carpet and I pretend that it looks nice. It does, sort of.
8) I have not had really good luck with the sulphur denitrators and I am not using them at the moment. They always seemed to keep the ammonia at 0.5 ppm instead of zero, and the nitrates around 10 ppm. Maybe in some ways this was okay, especially since I go for South American tanks so my water is slightly acid. Anyway it was not what I wanted. I wanted that 0-0-0. So I stopped using them.