The water in the bag could be significantly different from your tap water as it's not water from the shop round the corner. Some members will advise slowly adding your tap water to the bag; others will say that it days if not weeks for a fish to acclimatise to different water so you may as well just remove her from the bag straight into her new home. I'm not sure which is the best approach.
As for acclimitising, whenever I have bought fish or shrimp in the past I have always floated the bag in the tank and very slowly poured tiny amounts of water into the bag over the course of a few hours to give the fish time to adjust, then I would tip some of the water out of the bag and into the tank, keeping the fish in the bag with some remaining water, and then start to pour in more, little by little, until I felt it had been long enough, and pretty much all the water had been mixed down, only then would I release the fish into the tank. I would expect this process to take up to 6 hours or more, might sound like overkill but I think it works.
Acclimatising new livestock, there has been a lot of debate over this that I've seen over the years.
From what I've learned from a couple of very knowledgeable members on this forum that acclimisation livestock should ever really only be for wild caught fish from low ph waters of say 4.5 or less going into a home aquarium where ph can vary from 6.5 to 8.5 so only folks that have a wide knowledge about acclimation should really attempt to do this as it does indeed take weeks if not months to properly acclimatise wild fish.
However, for our perspective of putting in LFS or online store bought fish where fish are used to roughly the same ph as in our aquariums, the only acclimatisation one needs to do is for temperature matching the water as the water in the bag where livestock is will undoubtedly be cooler by the time the bag with water and livestock arrives at your home, so slowly increasing the temperature of the bag water to match your tank water by floating it on water surface and perhaps adding some tank water to bag if you wish to do so, this helps to stop temperature shock in livestock. This does not take long to do so really.
But if online store bought livestock is travelling over a longer period of time, say at least 3 hours to a couple of days, then a longer temperature matching period may be required, but do bear in mind the fish has been stressed and has been pooping and breathing ammonia into that little amount of water in the bag, its probably unpleasant for the livestock so do this as quick as you possibly can but not rushing so much it makes things worse.
I've seen the effect of temperature shock in livestock, its not pretty, bent spine and swimming erratically etc.
BUT, if the store is only half an hour to an hour away and you've gone straight from the store to your home, simply add the fish to the aquarium straightaway, net the fish and add to tank, NO bag water is to go into the tank at all under any circumstances.
Adding bag water from LFS to your tank is potentially asking for trouble as you have no idea what is in the water from LFS.
Most LFS runs a single water course that runs through all their tanks in the store, this makes perfect time management and business sense to do so. Not all LFS do this but most do.
But because the LFS is potentially running all tanks from one source, and water runs through all the other tanks, the water may collect diseases and unwanted elements such as algae, debris and lord knows else could be in that water, you do not want to add this to your tank and thus start affecting all your livestock.
For most folks who have done this and found it is ok in most cases to add a little water from bag to tank but its just not worth it imho as the risk of having something spreading through the water column in your tank is too risky for me to ever do this.
