Since the obvious things haven't been mentioned yet, I'll do so here!
Firstly, when plants die, they rot, and the first sign will be holes in the leaves. Some plants go into "shock" when transported, primarily because their roots have been broken off when they were moved, or else leaves were snapped and die. Vallisneria and Cryptocoryne are both prone to this.
Land plants simply die underwater; unfortunately, large numbers of land plants are sold to inexperienced aquarists: Dracaena, small parlour palms, Syngonium, Chlorophytum, purple waffle, and various other species. No matter what you do, these plants will inevitably die underwater. The leaves rot, and often you'll notice the tough fibres left behind after the green cells have rotted away. Most anything that looks like a land plant probably is a land plant, and the way to test is simple: take the plant out of the water, and if the leaves stay up, it's a land plant, or at least an amphibious one (such as Anubias).
Herbivorous fish will eat soft plants. Many more fish are herbivores than some aquarists assume. Even supposedly carnivorous fish such as puffers and oscars eat significant amounts of plant material in the wild. Loaches certainly do eat plants, particularly clowns and other large botiines. Snails tend to be highly herbivorous, with a few exceptions such as deposit-feeding Malayan livebearing snails and pure carnivore species such as Clea helena. Virtually all crustaceans are herbivores, though shrimps generally do little damage because of their size. Crabs and especially crayfish, on the other hand, feed almost entirely on plant matter in the wild, and can be highly destructive in planted aquaria.
The best plants to use in tanks with herbivores are Anubias and Java ferns, though the big Vallisneria and Crinum species usually do well too. Personally, I find Amazon swords too easily damaged (snapped) to mix with boisterous fish. Anubias and Java ferns also work great for another reason: they are epiphytes, so you don't put them in the substrate. This means they won't be uprooted. To be fair on giant Vallisneria species though, once these plants are established, their root systems are so massive that only the biggest fish can possibly budge them.
Plants also vary in how they react to water movement. Some thrive in high water movement environments, being adapted to such; Vallisneria for example thrives in fast water habitats. Others are swamp dwellers, and can be damaged by persistent buffeting; Bacopa for example is a brittle pond/marsh plant. If you find your plants are losing leaves or having their stems broken, review this aspect of their ecology and move them as required.
In a very general sense, plants will only do well given enough light. This is -- by far -- the single biggest reason plants die. You don't need a fancy substrate and you certainly don't need CO2 to have healthy plants. But you must give them plenty of light. Even a bright tank is giving them 'starvation rations' compared to the Sun, and for light-hungry plants it is almost always required to add additional lights to those that come with the aquarium. While the details vary, you can't go far wrong starting with 2 watts per gallon and choosing mid and low light plants such as Cryptocoryne, Anubias, Vallisneria and Hygrophila species. Cryptocoryne and Anubias are both very reliable and some Cryptocoryne at least are hardy and will tolerate lower levels of light. Vallisneria and Hygrophila prefer strong light, but will do okay at medium light levels. Hygrophila won't grow at all in dimly lit tanks and slowly rots away.
Within reason, dead plant material will not cause water quality problems. Plants don't contain much protein, using carbohydrate-based chemicals such as cellulose for many of the things animals do with protein. Consequently, when they decay, they don't produce much ammonia. This is why it's safe to leave carrots and such in tanks when feeding loricariid catfish; if you did the same thing with chunks of fish meat, you'd end up with an ammonia spike.
The main issue with dead plant material is that it can clog filter inlets and gum up biological media. Over time, this can reduce the efficiency of your filter, leading to circulation and water quality problems. So it's a good idea to clean away dead leaves and things at least once a week.
Cheers, Neale
