the only critter i know that will solve your problem are harlequin shrimps, and they arent easy to find locally. i could be wrong though about regular hermits eating them. i used to have a handfull of them and within 2 years only 1 lived. they just kill eachother off i think. i would only look into adding one seeing as how on most forums it says they are very effective in eating astreas. so what youd want and be happy with is a nice equilibrium where you have only a few left while not worrying about buying them to keep feeding your harlequin.