There are several diseases that are dangerous to humans that are sometimes found in tanks, hemoragic septicimia, fish tuberculosus and Pfeisteria are some, here is a blurb for you Chesapeake Bay people who keep brackish natives,
Broad, W.J. (1997). Battling the cell from hell. National Wildlife, 35(5): 10.
Pfiesteria piscicida is a microbe discovered by scientist JoAnn M. Burkholder that is very dangerous. Pfiesteria has been known to wipe out rivers by killing huge numbers of fish, and it causes nausea, disorientation and memory loss in humans.
The memory loss can be accompanies by lowered IQs and loss of coordination and is permanent