Why cant fish be directly added to a new fish tank?
In: Biology
If you’re referring to the practice of keeping the fish in its original bag for a half hour before opening it up to the tank, it’s purely to equilibrate the water temperature so the fish doesn’t suffer any thermal shock.
Fish are sensitive to sudden changes in water temperature and chemistry. Some fish (and inverts) will die rather quickly if not acclimated properly.
There are a few ways to acclimate. Floating the bag in the tank water for a bit and then mixing the two before putting the fish in equalizes the temperature and chemistry, but could still be too fast. Drip acclimation is a slower but better process where you siphon water from the tank into the bag through slow drips over several hours.
Ideally, before you even get to the point of buying the fish and acclimating, you’ve cycled the tank. You set everything up (substrate, plants, decor, filter, and heater (if tropical species)) and then add a small amount of ammonia (you could also use pieces of fish food or something like uncooked shrimp) with some self start bacteria; although, the bottled bacteria can be hit or miss based on how old the bottle is and how it’s stored.
You then have to monitor the levels of ammonia, nitrate, and nitrate until you get 0, 0, and some amount of nitrate. This signifies you have the necessary bacteria to turn ammonia from the fish into nitrite (which is slightly less harmful) and then to nitrate (which is the least harmful of the three).
Once cycled you can start adding fish, but you have to be careful about how much you add at once. Ammonia can spike if you add more fish than the bacteria can handle. You also need to watch your water parameters and do water weekly or biweekly changes (depending on your nitrate levels) to keep the water safe for fish.
If the tank is not cycled before adding fish, the risk of mortality is higher. Technically, you can do fish in cycling (where the fish produce the ammonia to grow the bacteria) but it’s more stressful on the fish and requires multiple water changes per week to keep the water parameters in a safe range. This is why tanks collapse when people add fish with no knowledge of cycling (aka new tank syndrome).
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