oh there's tons, but a lot work on similar principles
Carbon dosing is one of the major ones, which can be system wide using simple easy to make solutions of either ethanol (usually vodka), sugar, vinegar, or some combination of all three. There's hundreds of threads on reef central and a bunch of articles written on how to do it, how to ramp it up, etc.
Then there's solid carbon dosing which usually means a biodegradable carbon polymer like biopellets tumbled in a reactor, though a newish product that doesn't require a reactor has come out of South Africa recently, and is the same polymer but formed in block that looks like feta cheese (never tried that one).
The logic of carbon dosing being built on the redfield ratio, which finds that ocean going plankton contain C:N:P molecules in the ratio of 106:16:1, meaning for every molecule of nitrogen consumed, 6.6ish molecules of organic carbon are also consumed. It's based off of measurements of pytoplankton, and in reality should be considered a general average (the specifics are always more nuanced than that), but aquarists have extended it potentially apply to heterotrophic bacteria as well and hypothesized that from a bacteria's point of view our tanks are organic carbon limited. Adding organic carbon in excess, so the theory goes, will allow excessive growth of heterotrophic bacteria that will consume large quantities of nitrate and some phosphate (in a ratio of 16 to 1), and those bacteria can then be either consumed by corals or skimmed out by a skimmer (hence why most suggest pointing the outflow of a BP reactor at the intake of a skimmer).
Carbon dosing has it's risks, benefits, proponents, and adamant detractors. There's hundreds of threads on all the forums about it. The risk, is that cyanobacteria assemblages also contain clades of heterotrophic bacteria which are just as good (if not better because of their commensal associations) at consuming excess organic carbon, so if cyano is present and nitrate is high, it's possible to cause a cyano explosion by starting an organic carbon dosing regiment. Paradoxically, it's exactly the effect the method is attempting produce (cyano is a bacteria that consumes lots and lots of nutrients after all), it's just not the right effect. The challenge with carbon dosing is getting the heterotrophic bacteria you can't see as a gross red slime covering everything to become dominant, then managing it in such a way that leaves enough nutrient in the water for your corals. That's not always easy to do, which is where I think a lot of the detractors come from.
Then there's systems like prodibio, zeovit, and bright well aquatic's version of zeovit. Prodibio is a probiotic system that is supposed to encourage beneficial heterotrophic bacteria that consume nutrients, as is zeovit and brightwell, only those last two also include a zeolitic substrate that's supposed to both absorb certain nutrients directly from the water as well as provide a substrate for the bacteria you want. You just have to be careful with them as they each contain as part of their core 'regiment' the dosing of an organic carbon source, only they're not nice enough to tell you on the bottles that that's what you're dosing.
Then there's things like sulphur denitrators, or other denitrator reactors that create anoxic conditions inside them to favour and feed the denitrifying bacteria that break nitrate down in to atmospheric nitrogen. Those are sort of an older technology that never really caught on in the general public for all sorts of reasons, not the least of which being the potential for them to go horribly wrong and dose your tank with hydrogen sulphide.
And then really old school are properly designed deep sand beds, which a lot of people on here wouldn't touch with a 10 foot pole. However, the original 'inventor' of a deep sand bed specified the use of a plenum (a void created by some permeable structure in the bottom of the sand bed) in conjunction with the sand bed that most people in modern times seem to forgo, but to me seems critical to the design
There might be other ways, and each one of the ones I listed all have people who love them, hate them, think they should be banned, and can't understand why everyone doesn't use them. There's really no right way, and each one warrants investigation so you can get a sense of how they work and what they're doing. Always keep in mind that people on forums (myself very much included) often speak in absolutes as though they know what's happening, when in reality we're all just groping in the dark and are all equally as guilty of thinking we know more about causal relationships than we really do. It's as much an art as it is a science.
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