I need a snail kind and a shrimp kind that is large enough not to be eaten by my 5 Black Ruby Barb juveniles and that will clean the 20 gallon tank from brown, green and yellow algae. Also, I have no lids on the aquarium and based on the kind of filter I have.. I don't plan on adding a lid unless it's a net that covers the top. My barbs are semi-aggressive. They eat bladder snails and they eat red cherry shrimp. I need something large enough to intimidate them, that also won't escape.