Basics : Fish Introduction
Imagine you live your life in an apartment sized box (which you never leave), and every day a giant comes along, removes part of the ceiling and drops food in, but sometimes - they drop another person in, to share your space, your food, maybe even to compete for your mate. The new arrival might come from your part of the world, but more likely they have come from someplace completely foreign, with customs and behavior which are completely unknown to you. Does this sound like a fish's perspective? If you are very sociable, then you might like having a new roommate, provided they don't try to eat you. Some typical reactions might be curiosity (are they edible?), excitement (finally, some new action) and some apprehension, and if you (or they) are territorial, then look out.
Assuming your new additions are reasonably compatible, you want to make the introduction to be as low stress as possible. Don't have too many fish in your fishbag, open and roll the sides back and periodically add tank water until the floating bag is close to tipping. After feeding the current tank occupants, release the new fish into the back of the tank (towards shelter), and consider doing this all under reduced light levels (especially when mixing semi-compatible fish or territorial fish). Re-arranging the interior before introduction will often reset the pecking order, allowing a new arrival to safely slip into a spot low in the order.
In details, I discuss the sequence of introduction, some bag floating techniques, bagging and transport, and water shock.
Details...
SEQUENCE OF INTRODUCTION
When stocking a new tank, start with the meekest and smallest fish. This will give the most vulnerable fish, the best advantage, by knowing all the hiding places and having the home court advantage. Then proceed to add progressively larger or more territorial fish. If your arrivals fall out of sequence, then 1) feed the community (fish with full stomachs do not swim as fast, or have as much interest in chasing something), 2) if there are territorial considerations, re-arrange the interior of the tank, maybe even adding a new plant or rock somewhere, 3) turn off the lights (everyone winds down), and release the new fish into the back of the tank (they can spend the night poking around discovering shelter). Fish are much more amicable with accidental discoveries of a new fish in the morning, probably wondering if that fish was new.
Walking to your tank, opening the cover, floating a bag and then dropping a fish in, is probably the worst way to do it, when carnivores are mixed with community fish. It is just too close to a feeding sequence, except you made them wait for the food, and the food just happens to be swimming today. Many fish will have a go at what they think is a new food offering, and damage or death can result. Sneaking the fish into a corner not used for food, under low light conditions can be much more successful and less stressful.
FLOATING
In a fishbag, fish wastes and elevated CO2 levels will both cause the pH to drop causing some stress (not to mention being bounced around in a fish bag). Floating a sealed bag will usually increase the temperature (reducing the amount of O2). However, opening the bag will dissipate the CO2, causing the pH to climb, which causes the ammonia waste to convert from non-toxic NH4 to toxic NH3. There isn't a perfect way to do it, so that's why I'm pointing out all the nuances of different actions. Additionally the hobbyist has to contend with how the fish will be acclimated to the usually different water chemistry, from the tank he was last in, to where he is going now. Being moved is one of the most stressful conditions fish regularly encounter, and stress weakens fish, making them susceptible to diseases.
A good compromise is to open the bag and float it with the sides rolled back (I put in a few drops of Ammo-lock to neutralize any ammonia). After a few minutes, take a 1/2 cup of your tank water and pour it into the bag. Repeat this again every 10 minutes until the bag is very full, and not able to float properly. Then net the fish out and into your tank. Use the advance feeding and the cover of darkness if the situation calls for it. If all the fish are community fish of comparable sizes, then it's not a requirement.
BAGGING
Considering the chemical activity going on in a fishbag, it's a good idea to distribute your fish lightly for longer trips (one per bag for medium sized fish). A styrofoam box is very useful for fish transfers, and is often available from pet shops by just asking for it. Dollar stores sometimes sell lined shoe purses (to keep shoes warm in your car during winter commutes to work). These are ideal for transporting fishbags without the trouble of a large styrofoam box.
A professional method, (useful to know, especially when acclimating expensive fish) is to open the fishbag into a pail on the floor in front of the destination tank. Take water sample from pail and connect a drip line, gravity feeding from tank into pail. Check water parameters and act accordingly, sometimes moving the fish and drip line to a different tank closer to the bag parameters, such as when pH is found to be higher than expected. When the pail is full, the bag water has been diluted by a factor of 3 or 4 and the fish can be transferred.
Fishbag water is not normally used. If there is any contagions, the fish will still be carrying it, but there is no need to add more with the fishbag water. The only time fishbag water is used is when you need it to do a more gradual acclimation in a small tank, such as a quarantine tank.
WATER SHOCK
Ordinarily, try to keep source and destination water within 0.5pH and 40ppm NO3 from each other, and less (20ppm NO3) for smaller fishes. Nitrate shock (NO3) manifests itself within approximately 3-5 days (for larger fish), sooner for smaller fish. Large fish often recover, usually after a bout with Ich. Smaller fish will randomly survive NO3 shock or not.
Fish should never be moved from a zero NH3/4 or zero NO2 environment into a positive NH3/4 or NO2 environment. Fish have some ability to adapt to levels of toxins which have increased slowly, but significantly less immunity when they encounter a sudden change upwards.
Rapid changes in water hardness can also be very stressful. Moving from very soft water to very hard water causes temporary respiratory stress due to the different osmotic pressures on the gill cells, however, moving from very hard water to very soft water can cause the same cells to explode, causing permanent damage.
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