Just a quick question if I ask my lfs for some spare media will it still be "live" when I get it back to my tank ?
Hello; As with most things it depends. Time and staying wet likely being the two key things to take care about. I would bring my own plastic bag. Ask that the media or whatever they offer be packed in water and put it in my new setup within hours and not days.
In other words don't get a bag of gravel, sand, old filter media, tank ornaments, sponge filter base, live plants, snails, bio balls or whatever and let it sit around several days before putting it in the new tank setup.
I actually do not know how long the bb will stay good in a closed bag of water. My best SWAG being that as the length of time increases then the amount of viable bb decreases.
A couple of things to keep in mind. First the bb are alive and essentially some feed on ammonia and others feed on nitrites. There are at least two major groups of beneficial bacteria. If you get the bb home alive and well and into the new tank setup, they will be hungry. The ones that feed on ammonia will convert ammonia into nitrites so they will feed the second group. You will need to give a source of ammonia for the first group.
In a tank setup with fish it is the urine and other waste byproducts from the fish that provide the ammonia. Fish make ammonia as a waste byproduct. Some bb take up that ammonia in their life processes, change it into nitrites and excrete the nitrites. Other bb take up the nitrites, change and excrete it as nitrates. So do not put the bb loaded stuff into a tank that does not have a source of ammonia.
There are sources of ammonia other than waste from fish. I use snails in all my tanks except fry (newly hatched) tanks. The snails do two things. first they will have bb on their shells. Remember bb coat solid surfaces as a bio film. Next the snails are alive and produce ammonia themselves as a waste byproduct which will feed the bb on their shells. For folks who like snails it is a win win. If you happen to not want snails o be permanent there is a type that will not reproduce in fresh water but will live nicely in it, called niterite ( my spelling is wrong). There are other sources of ammonia in addition to fish and snails.
The next thing to keep in mind is the dynamic balance between the mass of fish in a tank and the size of the bb population. Over time the population of bb in a tank will match the amount of their needed nutrient (ammonia) available. If you add a very large fish ( or several small) to a tank already in balance that fish will become a new source of ammonia. The existing bb will not have the numbers to "eat" all that new ammonia immediately. They will start reproducing right away and will eventually make enough new bb to handle the extra ammonia. What I am getting at is in a new tank do not add too many fish all at once.
I will pause here as there is way too much information for one post. You have a very steep learning curve ahead of you.
Good luck