I only give a quarter because they are for my snails and they don't eat much.Small amounts of the algae wafer is fine and even healthy to a degree, however, Betta will overeat on the algae wafer and this can cause problems.
It is better to not allow them to eat too much-you may have to find a different way to feed what ever you are feeding the algae wafers for.
This is one of the issues you can sometimes have with mixed species kept with Betta....Feeding.....Betta can be such pigs....lol.....and can eat themselves to death if your not careful......