To Float or Not To Float?

for the week of 5/7/98

Ever since plastic bags have been used to transport fish, floating these bags in the aquarium upon arrival has been a common practice. In theory at least, floating the bag of fish for a time can reduce stress from sudden change in water temperature, and if a bit of water is added from the tank periodically, stress from drastic chemical change can be reduced as well.

What we rarely consider, however, is that the very act of floating fish can be stressful! Gas exchange through the plastic, which is pretty limited in the first place, is greatly reduced when the bag is wet on the outside. In addition, fish often panic in such situations, able to see safe harbor in their soon-to-be home, but unable to get there. Finally, in some cases, fish have been in the confines of a small bag full of fouled water too long already, and floating only extends their misery.

In many cases, floating is probably unnecessary: the difference in temperature and major chemistry (pH, salinity, hardness and nitrogen compounds) between the tank of origin and destination is too little to be concerned about. In others, the differences are so extreme that floating and even float-and-dipping is inadequate, and a much slower acclimation process (such as putting the fish into an aerated container and slowly dripping tank water in) should be used.

Floating is probably best used in circumstances where the temperature difference is more than 3 or 4 degrees Fahrenheit. And even then, the process should be brief: a small bag of water can make up a 15 degree temperature difference in about ten minutes. Dipping can be incorporated if some chemical parameter are close, but not quite identical, but even more care must be taken. It is especially important to either refill the bag with air and reseal it or in some other way arrange for a large area of contact between water and air. Do not, as we are all tempted to do, simply hang the bag over the tank edge and secure it in place with the aquarium cover. This reduces the water/air surface area inside the bag to only a few square centimeters, and regularly leads to death or damage due to asphyxiation.

Finally, attention should be paid whenever fish are being acclimated. It’s amazing how often we hear, “We went straight home to float the fish, then went to a movie. And when we got back, the fish were all dying in the bags!” Even if it were necessary to float fish for so long – and it rarely is – it would be unwise to leave them unattended.

Submitted by: Jim Kostich

“Tip of the week” appeared regularly in 1999 and 2000.