1. every fish in the lot multiplies the chance that SOMEBODY will come in with something noxious and spread the gift to everybody. It's like That Kid who gives measles to the whole first grade class.
2. your newly-cycled (we presume) sandbed is doing well to support a handful of crabs and snails. ONE fish added is a major load. Two is twice that. You can end up pushing your infant sandbed too hard too fast.
3. water resources in the typical 10-20 gallon qt tank are sparse enough, re oxygen, clean water, and bacteria able to process waste. If you are running more than one fish for any reason, be doubly watchful of those resources, and have a plan to oxygenate, do a water change, and a Prime dose ASAP. Understand that if you do get an oxygen crisis, fish will be close to the top, but are you going to be able to watch that tank at night as well? Ammonia will not announce itself: have an Ammonia Badge, test, and be very careful about this. Ammonia usually kills with about a three day lag, through damage to kidneys. It is quite sneaky.
2. your newly-cycled (we presume) sandbed is doing well to support a handful of crabs and snails. ONE fish added is a major load. Two is twice that. You can end up pushing your infant sandbed too hard too fast.
3. water resources in the typical 10-20 gallon qt tank are sparse enough, re oxygen, clean water, and bacteria able to process waste. If you are running more than one fish for any reason, be doubly watchful of those resources, and have a plan to oxygenate, do a water change, and a Prime dose ASAP. Understand that if you do get an oxygen crisis, fish will be close to the top, but are you going to be able to watch that tank at night as well? Ammonia will not announce itself: have an Ammonia Badge, test, and be very careful about this. Ammonia usually kills with about a three day lag, through damage to kidneys. It is quite sneaky.