you don't need to remove the fish.
1. Gravel vaccuum your present substrate THOROUGHLY. Do half of it one day another half the next and then repeat that again. Or you can do it all at once by putting the water through a towel to remove crud and then putting it back in the tank.
2. Rinse your sand really well. I do this by placing a 1" layer of sand in the bottom of a bucket (5 gallon tanks work well) then I fill the bucket with a water hose all the while stirring the sand with the flow from the hose. When the bucket is full I dump out all the water leaving only the heavy sand particles in the bottom. I repeat this process until the water clears to crystal within ten seconds. You can take this as far as you want or not rinse at all. It all depends on how much cloudy water bothers you. It shouldn't hurt your fish. One benefit of rinsing very well is that if you have a magnetic driven impeller on your filter and you don't rinse your sand, then you will have to turn it off while putting the sand in your tank and leave it off until the cloudiness settles. The sand grains can clog your impeller and make horrible noises or break your filter. How long you will have to let the sand settle if you don't rinse is beyond me. I've heard from anywhere between 3 days to a week. Oh yeah one more thing. I think pool filter sand is suppose to produce minimal cloudiness.
3. (you may want to remove any decoration you may have in the tank at this point...it's up to you)After you have rinsed your sand you need to remove your gravel. This is the most dangerous part for a few reasons.
A. If you have any khuli loaches or some other critter that likes to hide in the substrate you might accidentally catch and kill the little guy.
B. If you have snails again you might catch and kill them.
C. If you have not thouroughly siphoned your gravel you may stir up a pocket of gas produced by anaerobic bacteria. This can (although rarely does) kill your fish. Even if your fish survive it smells like rotten eggs and we don't want that right.
3a. The easiest way to remove Gravel IME (in my Experience) is to use a fishnet. Just scoop it out and sort through it to make sure you aren't losing any snails (i'll tell you why you want snails shortly) Then dump the gravel wherever your going to store it.
4. Now it's time to put the sand in the tank. Simply scoop up a cupfull (coffemugs work well) of prerinsed sand and lower it into your tank. Don't dump it in... Just slowly lower it to the bottom and dump it in a corneras far from the filter intake as possible. If you've rinsed well (clears in 10 seconds) then you do not need to turn off your filter for this...if you haven't then you should definitly consider turning off your filter for a while.
5. Now we get into caring for sand. It's a bit tricky compared to gravel. First of all you will need to clean it weekly. You can do this by hovering your gravel vaccum about 1/2-1" above the sand. It will suck up all the poo and leave (the vast majority of) the sand alone.
6. Next you need to think about anaerobic bacteria. Anaerobic Bacteria love settled sand. Anaerobic means without oxygen. These bacteria create black spots and stinky gases. You can eliminate these bacteria a couple different ways.
A. Stir the sand weekly by hand...pretty self explanatory
B. Get some critters to do the work for you. There are lot's of fish and inverts that will sift your sand for you. The best qualified critter is an MTS (malaysian trumpet snail) These are easily aquired. Just go into your LFS (local fish store) and ask them for some pest snails. They will probably give them to you free. Make sure you get only MTS snails though. You can look them up quite easily. They are the ones shaped like cones. Other critters that can perform this task include khuli loaches and cory cats. There are lots more like shelldwellers and some puffers but you'll have to look into that on your own.
Well I hope this has been helpful. good luck.
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