Nitrates and phosphates are main nutrients for algae. When one or both nutrients are withdrawn from the water, the algae stop growing! 
However, bacteria only break down nitrate under specific conditions: Only when there is no longer any oxygen, and a carbon source (a nutrient for bacteria) is available do the bacteria remove the oxygen from the nitrate, thus breaking it down. 

That’s how JBL BioNitratEx works: 
Bacteria settle on the surface of the JBL BioNitratEx bio balls within about 4 weeks and form a thick biofilm. Beneath the biofilm a low-oxygen zone forms, in which the bacteria start the denitrification process to degrade the algae-promoting nitrate (NO3). The bio balls serve as food and the nitrate serves as oxygen supplier for the bacteria. That’s why the bio balls become smaller as time goes by (easy to check visually) and disappear completely. This takes about 6-12 months, depending on the nitrate pollution. Then you need to replace the bio balls. 

Snails: If lots of snails reach the JBL BioNitratEx balls they will eat the biofilms from the balls’ surface. As a result oxygen will reenter the lowest bacterial layers and the bacteria will no longer need to dissolve the oxygen from the nitrate to break it down with it. If you have a lot of snails it is better to put the JBL BioNitrat Ex balls into a mesh bag to block the access for the snails. 

The difference to JBL NitratEx: 
JBL BioNitratEx works on a purely biological basis through bacterial nitrate breakdown. In contrast JBL NitratEx is a synthetic resin which binds nitrate from the water (only freshwater!) and replaces it by harmless chloride (ion exchanger function principle). The advantage of this system is that it works quickly and you can use it to remove nitrate from tap or well water etc.