Old, mature tanks that house one or more herbivorous fish can hardly even support many snails. Your LR is practically covered every inch with coral, coralline algae, sponge and other reef organisms so there isn't much room for nuisance algae. A massive growth of Green Star Polyps covers the upper half of the back of the tank! Phosphate reducing media is also used. Parameters have been kept within range and the lights are the some of the best LED fixtures, designed to support coral while not supporting nuisance algae. That tank has always been fantastic. It is the major feature in your lobby and its awesomeness has spoiled your co-workers to anything less.

Aloha,

Mark 

It can be difficult to find the right balance. Because each tank is different, it takes a lot of experience and skill to achieve that balance, especially in a new tank. Snails must have some algae to eat. They should not be added unless/until algae is starting to show and then the number added should be only as many as it takes to keep algae controlled. Sometimes just a couple snails at a time. They should be added in steps. 

No matter how many snails, the glass is still going to need scraping. Snails don't do windows. 

When a snail dies it should be removed immediately, otherwise it has the opposite effect, adding nutrients that increase algae growth. Speaking of nutrients, the use of Phosphate removal media, either AA or GFO has become common, even in new tanks. This media inhibits and in some cases even stops algae growth, leaving nothing for Snails to eat.