It is very important to properly adapt discus fish to their new environment after transportation.
This relates in particular to the adjustment of the water temperature.
Since fish are poikilothermic animals whose body temperature is determined by the ambient temperature, the animals should not be exposed to large temperature differences, as they can otherwise be damaged and in the worst case even die directly or in the next few days from the temperature shock.
As the transport water cools more or less depending on the season when the fish are sent, the temperature of the aquarium water must also be reduced beforehand to keep the temperature difference as low as possible.
I recommend lowering the water temperature in the aquarium at outside temperaturef from -5 ° to 0 ° C to 21 ° C, at outside temperatures from 0 ° C to 5 ° C to 22 ° C, at 5 ° C to 10 ° C to 23 ° C and at temperatures from over 10 ° C to 24 ° C.
Depending on the season, the fish arrive at temperatures between around 17 ° C and 24 ° C.
You should measure this temperature. Today, this is done contactlessly with an infrared thermometer. If you do not have such a thermometer, you can also put a normal aquarium thermometer in the styrofoam box between the closed fish bags and close the box again for 5 minutes. You can then read the temperature.
If the difference in water temperatures is only 2 ° C, the fish can be placed directly in the aquarium without adjusting the temperature (without transport water).
Up to a difference of approx. 4 ° C between the transport water and the aquarium water, the fish can be adjusted to the temperature of the aquarium water within 30 minutes. It is important that you do not open the bags as long as the temperature has not yet reached the aquarium temperature up to about 2 ° C. The oxygen content in the transport water would otherwise decrease relatively quickly. Even after 3 days there is still enough oxygen in the bags, so that opening the bags hastily does more harm than good.
If the temperature of the transport water is more than 2 ° C lower than that of the aquarium water, the temperature must be adjusted.
There are several options or a combination of these.
In the first method, fill the transport box about half with water from the aquarium and put the still closed bags back in. After about 10 minutes, fill the box almost completely and wait another 10 minutes.
Then empty the box and measure the temperature of the transport water again. If the difference between transport water and aquarium water is still greater than 2 ° C, repeat the entire procedure until you measure a difference of only 2 ° C.
When this is done, open the transport bags and take the fish out by hand and put them in the aquarium. Under no circumstances put aquarium water into the transport bag to adjust the water values. Fish tolerate rapid changes in water parameters such as pH and conductivity as long as you are in the usual aquaristic areas. Only the temperature should be carefully adjusted.
If you put aquarium water in the transport bags, the fish could die of ammonia poisoning under unfavorable conditions or at least suffer considerable damage to their gills.
In the second method, which can only be used if you do not have any fish in the aquarium to be populated with the new fish, you first measure the temperature of the transport water and then lower the temperature of the aquarium water by changing it with cold water as long, as you measure nearly the same temperature in the aquarium as in the bags. Again, the bags are only opened when the temperature has adjusted. After opening the bags, the fish are carefully removed and placed in the aquarium.
The transport water is very heavily loaded with metabolic products and should not be dumped into the aquarium.
Of course, you can also combine both methods.
There are also other methods, such as filling the transport box or a large bucket with tap water at the same temperature as the transport water. Put the fish in there (without transport water) and aerate the water so that the fish do not suffer from a lack of oxygen. Then slowly warm the water up to the temperature of the aquarium. This is best done by mixing with water from the aquarium (however, this must not be too heavily organically contaminated, as otherwise there is a risk of ammonia poisoning again), or with a little warmer tap water.
In my experience, fish can be adjusted within 30 minutes up to a difference in water temperatures of around 4 ° C.
For each additional degree, you should allow yourself another 30 minutes.
The adjustment should take a little longer than too short.
If you adjust the temperature of the fish using the above methods, nothing will happen to them and you will enjoy them a lot.