How many sw
tanks does he have? Sounds like a bit of old world thinking to me. I remember years ago when I first got interest in sw
, it was common and expected that the fish will only live a short time. That's not the case anymore though. We have more knowledge and know better now how to care for them. These fish live for years.
I see nothing wrong with what you have. The crabs are benefitial and will not kill your tank. If anything, I don't think you have enough crabs and snails but more can be added as needed. Keep on reading and teaching "him" how to properly maintain a sw
tank. Good luck.