I have the same tank and about a month back put a small one in. It was the size of a half dollar, found him dead a few days after I put him in. My levels were all zero (nitrates, ammonia, nitrites) he was stuck on my Mp10 don't know if he was weak and got sucked or died and then got sucked in. I keep the pump low around 25% on lagoon setting.
If you can find a dime sized one or around that area, you should be set for awhile.