Because the capacity of the aquarium is governed by its size, it stands to reason that a smaller tank must hold proportionally fewer animals. The parasite is thus faced with the same ratio of animal mass to tank volume as it would be in a larger tank. The fact is, if the capacity of any tank is exceeded, there is a greater chance of a parasite finding a host and a much greater likelihood of the water quality deteriorating. Water temperatures are less stable in a small aquarium. This is perhaps a minor point, because the tank temperature can drop only to the room temperature, and this is not fatal to most organisms unless the temperature remains low for a substantial amount of time.
Evaporation can be more of a problem in a small tank.
Even when tanks have relatively close-fitting glass covers, there is some evaporation of water, which increases the salinity of the aquarium water. In a small tank, particularly in warm weather, especially if the tank lights are on, the amount of evaporation can be significant. If the tank is left unattended for several days or more, the salinity could increase to the point where the fish and invertebrates are under physical stress, possibly resulting in disease and even death. Temperature and salinity should not be a problem if the tank is monitored every day or two. This, in fact, may be the only real drawback to a small marine tank.