Gravel Change

I changed 3 tanks from gravel to sand a couple of years ago, so it is possible to do it without problem 


Unless you have an undergravel filter, there will be some bacteria in the gravel but the majority will be in the filter. I would take the fish out while you remove the old gravel as there is likely to be a lot of muck in there. If at all possible, remove as much water as you can and put the fish in it. Then remove the gravel and if the water that's left is very bad, remove that too. Put the new gravel in, decor, plants etc, replace the water and fish you took out then just top up with new water. I used a plastic dustbin when I did my 125 litre, but I had to completely empty it as I was also moving it across the room. If the gravel doesn't contain too much muck you'll be able to get away with leaving some water in the tank.
Wash the new gravel before you start. Feed the fish lightly for a day or two before and after to reduce the ammonia they produce, and keep an eye on your ammonia and nitrite for several days after (and do a water change if you see either of them).