It is mostly up to you to decide which is "bad". Personally, I use Malaysian Trumpet snail the most, but I have pond snail, ramshorn, mystery snail, and nerite snails in all of my tank.
MTS is great for churning the substrate to prevent gas build-up and get rid of small un-eaten food. MTS, ramshorn, and pond snails reproduce when you overfeed your tank, if you keep your feeding consistent, they eventually even out after a couple of months. 1 of my tank had 200 snails and now down to about 20 because the fish finish up all the food so there is none for the snail to reproduce.
Mystery snail is the most fun to look at, you can prevent them from breeding by having a high water level in the tank, they only lay eggs on the wall above the water if there's any.
Nerite wont reproduce in fresh water so you don't have to worry. they are known to be the best algae eater as well.
none of the snail will eat your plants or other inhabitant unless they are already dead so don't worry
This video explains the benefit pretty well. Starting at 1 min:
Agreed. It depends on what you consider as bad. Nerites are considered some of the best algae eaters. Bladder snails, ramshorns, mystery snails, and MTS will all eat leftover food/detritus and therefore will help reduce the amount of nutrients released into the water column for algae to use to grow. Rabbit snails are all different but in general those are more treated as pets than scavengers.
A lot of the above species are known to be plant safe, but they will eat dead/dying leaves, so it might look like they're eating your plants. But in reality they're just eating the dead matter away.
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