Ever since plastic bags have been used to transport aquarium fish, hobbyists from around the world have utilized a simple method to acclimate new arrivals. This process, commonly called "floating and dipping", can be quite effective in reducing stress in transported livestock if employed properly, but equally effective in causing stress if misused.

Why Acclimate?
As a general rule, fish in nature are not required to adapt to sudden changes in environment. A given area in a large body of water usually does not fluctuate chemically or in temperature in rapid order, and even when an abrupt shift does occur, fish can often relocate to an area more to their liking. As such, when fish are exposed to unavoidable severe change, whether in nature or an aquarium, the result is stress, which in turn may lead to illness and even death.

Acclimation serves to reduce such stress by allowing fish more time to adapt to changes in their environment.

When NOT to Float and Dip
Since the whole point of acclimation is to reduce stress, it should not be forgotten that the very acts of floating and dipping can be stressful. Fish often panic in such situations, able to see safe harbor in their soon-to-be home, but unable to get there. Also, 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.

There are two extremes of condition when it does not make sense to acclimate new arrivals by the float and dip method: when the shipping water parameters are nearly identical to the new tank water and when they are vastly different. If water temperature, salinity, pH and hardness levels nearly match, any stress caused by the acclimation process outweighs the stress saved. Fish can simply be netted carefully from the shipping bag and dipped into the aquarium. On the other hand, if one or more of these measurements is radically different, acclimation should take far longer than the fish can safely reside in the bag. In this case, the fish are best placed in a larger, aerated container, into which water from the aquarium can be dipped or continuously dripped over a period of hours or possibly days. In very rare circumstances, it may be necessary to adjust conditions in the receiving aquarium (preferably a quarantine or temporary holding tank) to match those of the incoming bags, then slowly re-acclimate to "normal" over a few days or weeks.

Float, Maybe; But No Dip!
There is yet another situation in which acclimation by any method is probably contraindicated. If fish are either overpacked or left in the shipping bag for an extended period of time, ammonia/ammonium levels can become very high. Oddly enough, however, carbon dioxide levels are often elevated as well, and the resulting low pH actually protects the fish by converting deadly ammonia into non-toxic ammonium. If the aquarist then adds water of a higher pH, aerates the water, or even leaves the bag open long enough for that carbon dioxide to escape, the ammonium is converted back to ammonia, and tragedy follows. Such bags of fish should receive no more than a brief period of floating - still sealed - and even that should be cut short if fish show signs of distress.

When TO Float and Dip
Float and Dip is a useful technique when the water temperature and chemical conditions of the transport bag and the aquarium are similar but not identical (see side bar for my rough guidelines). Most often, fish are acclimated after purchase from the local fish store, but a brief float and dip might be called for even when fish are moved to another tank in the same room. Any time the source and destination tanks differ substantially in temperature or chemistry, acclimation should be considered.

How Long to Float
In recent years, numerous aquarium articles have referred to a sealed plastic bag's ability to allow gas exchange by osmosis - an ability that is then apparently lost when the bag is wet on the outside. According to this explanation, oxygen molecules can slip in (and carbon dioxide out) between the dry bag's molecules sufficiently to keep a few small fish from suffocating. I've never stumbled across any scientific documentation of this effect, and I can't help but wonder if the increased metabolism of fish panicking in a floating bag has a much greater impact on dissolved oxygen levels. For one thing, the top half of a floating fish bag isn't particularly wet on either the inside or outside, so osmosis shouldn't be inhibited. Furthermore, fish that are double-bagged seem to have just as long a "shelf life" as those single-bagged - even if there is a substantial amount of water between the bags.

However, regardless of the cause, I can fully agree with the observation that a bag of fish that could have probably survived in a paper bag or box on the kitchen table for two days can show signs of respiratory distress after less than an hour of floating in the aquarium. The obvious implications here are that floating fish bags should be a short-term affair and that the process should not be left unattended. I'm often chagrined, but no longer surprised, to hear a customer report, "We rushed straight home and floated the fish, but when we got back from the movie, all the fish were dying in the bag!". Temperature equalization in particular occurs quite rapidly: a quart bag half full of water floating in a 30 gallon tank can pick up 15 degrees F. in a little over ten minutes.

Give 'Em Air!
Not only the duration, but also the technique of acclimation can lead to oxygen depletion. In particular, the tempting practice of draping an opened plastic bag over the edge of the aquarium - and possibly even securing it in place with the aquarium cover - can lead to asphyxiation within a very short period. It is crucial to remember that oxygen can only enter water in the few square inches where air and water are in contact. A typical 6 x 12 bag, sealed and floating on its side might afford 40 or more square inches of such "surface area", while the same bag opened but "cuffed" and floating vertically might provide 20. A collapsed or draped bag might result in only 1 or 2 square inches. Exactly how long fish will survive without damage depends on their size and quantity, as well as the initial quality of the bag water, but I have witnessed fish going from healthy to near death in as little as five minutes.

Mixing Water
Periodically during the floating time, water can be dipped up from the aquarium and mixed with the water in the bag. Adding about one-half the bag's original volume (e.g. adding a cup of aquarium water to the pint already in the bag) is equivalent to a one-third water change. The waters should be allowed to mix for a few seconds, then that same amount of water (one cup, in our example) should be poured off and discarded, as it is certainly not a positive addition to the aquarium and may contain excessive waste or infective agents. The bag should then be re-tied and floated horizontally again for optimum aeration. Three or four such additions, five minutes apart, is generally sufficient. The entire acclimation process can thus be completed within twenty or thirty minutes.

Releasing the Fish
A few minutes after the last addition of water, the fish should be gently netted from the bag and placed into the aquarium. The water remaining in the bag is again best discarded, rather than poured into the aquarium. While the float and dip is now complete, new additions should continue to be monitored closely for signs of shock or aggression from existing stock.

This article originally appeared in

Aquarium Fish Magazine

Copyright © 1999 James M. Kostich
All rights reserved.