i though that too being that i have a 40 gallon sump, but my bvn cleared the population in a month. i only have one and he was only ever in my display tank. i cannot find a single planaria anywhere in my tank, sump, or overflow (kinda sad bc now he’s starving). have a plan on how to decrease your flow specifically on the sand bed/ eliminate wide open areas on the sand bed by adding frag plug, run activated carbon to combat the bvn’s casual toxicity, and you’ll be fine. i run an ulns which is probably also helpful (carbon dose some vodka baybee it will change your life).