It is a parameters and time game. The first concern is how long the fish have been in the bag, how many fish and what size. Longer, more and bigger all magnify the issues Unless the temperature is radically different form that of the tank, minimal temperature acclimation should be needed. There are two much more important issues. TDS/hardness is one. The greater the difference between the bag water and the tank water, the more of an issue this can be. It has to do with osmoregulation. Most of the time this is not a huge consideration.
The most important issue is pH difference, but not because pf pH per se, but due to the effect pH has on ammonia. Travelling in a bag reduces the pH of the bag water over time. So the longer the fish have been in the bag, the more the pH of the bag water may have dropped. Ammonia causes water to become more acid and ammonia build up is greater the longer the fish are in the bag.
Where the greatest potential problem occurs, and what kills or harms most fish, is the process of acclimation itself. This happens when the water in the bag is of a lower pH than the water being added to the bag. In transport, even as the ammonia level in the bags is increasing, the pH is dropping and this in turn helps to render the ammonia less toxic. When we begin to add water to acclimate, it can often raise the pH. This can be the result of a couple of factors. One, if the new water is of a higher pH, this will raise the overall pH. But there is also the issue of the buffering capacity of the added water acting to raise the pH. Much the same as in a fishless cycle folks are told to keep the pH up by adding sodium bicarbonate, adding new water with a higher carbon/carbonate component can raise the pH. Thus acclimation can act to turn basically non-toxic ammonia in the bag water into more toxic water as the pH rises due to the addition of new water. Of course, if the bag water has a higher pH than the new water, this should not be an issue. One trick we can use, if we are worried about ammonia, is to add a bit of an ammonia detoxifier to the bag water- Amquel or Prime work fine ( I actually add a bit of Amquel to bag water when I ship fish.).
The above reason is why I hate when shippers add methylene blue or bag buddies to the water. It makes it virtually impossible to use common test kits to try and measure either pH or ammonia levels in the bag. Of course, if the bag water has a higher pH than the new water, this will not be an issue.
As a rule of thumb, the longer the fish have been in the bag, the greater the danger that acclimation techniques can cause more harm than good. Often bag water can get nasty enough that the risk of allowing the fish to remain in bag water is a much greater than the risk getting them into clean water ASAP. On the other hand, just bringing home fish from a local store or from a breeder or friend etc. where the fish are only in the bag for a few hours is much less of a risk. Moreover, fish shipped will have less water in the bag than ones hand carried- water is pricey to ship.
From a pure pH point of view, most fish can easily handle larger changes than most assume. I have seen wild caught Altum angels experience a change of close to 1 full pH point in under 5 minutes and suffer no ill effects. I have added muriatic acid to their water and done this myself. So I know it is possible and safe.
The final consideration is the specific fish involved. Different fish handle different types of parameter changes differently. While I have changed the pH for Altums by a full point, I am much more restrictive in terms of TDS changes. But for plecos it is the reverse. They handle big pH changes less well but can tolerate much greater changes in TDS as they are seasonal spawners and are able to to handle a rapid drop in TDS typical in the wild.
Another issue many folks ignore when floating fish in a bag is what is on the outside of the bag. Stores typically write the description or price on the outside of the bag, Then we plop the bag into the wtaer. How safe is that ink? And then how do we know what else may have gotten on the outside of the bag during the whole process. When buying in a store, ask the clerk to double bag and write on the outside bag which you can remove before floating. I also will run clean water over the bag before floating it to remove stuff that may be on it. Even better, when possible, is to unbag the fish into a container and avoid putting the bags into your tank. Another way to avoid contamination potential when you need to warm the bag water is to fill a bucket with warmer water and float the bag in the bucket and not in the tank. The nice thing is you don't have to dechlor the bucket water as the fish don't come into contact with it.
I used to do all sorts of stuff regarding acclimating new fish for a number of years. Now most of the time its a short temperature raising and then into a bucket or the tank. Sometimes I will do an addition of my water to the bucket before the fish get netted into the tank, but more often its bag to bucket to tank in under 5 minutes. Do not underestimate the benefits of moving fish into clean water sooner rather than later.
The trick to knowing what sort of acclimation is needed or not and what might be harmful or safe, you need to consider the specific situation- what fish, what temps, what pHs and what TDS may be involved and how long the fish have been in the bag.
