Well before you change the substrate, water changes need to be done weekly on your tank. At least 40-50%. That could be one of the big reasons he isn't swimming as much. Changing the water once a month is no good.
I forgot to ask in my last post, but what temperature is your tank set at?
BGK's are also nocturnal, and their eyes are super sensitive to light. So they need extra cover when you have the tank lights on.
As far as changing out the substrate, the best thing to do is get a couple people to help you do it. You're going to need a few 5 gallon buckets, you can get them at lowes or home depot for cheap. Get like... 4 of them.
Sand needs to be rinsed really well before you put it in your tank. Otherwise it will make the water super cloudy and may put toxic stuff in there. So pour some into a bucket, rinse it multiple times, then transfer it into a dry bucket. Repeat until all the sand is done with.
Next,
Fill each of the other buckets with water, and pull the decorations out. Give them a couple shakes to make sure they are clean, and put the ones you plan on putting back in there in a bucket with water.
Depending on what type of filter you have, either unplug the canister filter and leave it filled with water, or take the filter pads out of the hang on filter and put them in a bucket with the decorations.
Don't let the filter media dry out! Otherwise you'll have to re-cycle your tank.
Next, you want to CAREFULLY net the fish and put them in a bucket. If you have two heaters, I would put the BGK in one bucket and your other fish in another. He's so large that he'll hate the bucket, but he should fit. Your BGK probably won't fit in a net, so your best bet is to get your hands in the tank and lightly pick him up. Depending on how he is, getting him out with some of the decorations still in the tank might ne easier. (I've never moved my BGK, but that's how I would do it )
The next step is where multiple people come in handy so you can do this fast. Start draining the water and stir up all the gravel..you'll notice there will be a ton of debris and poop, so make sure you suck it up. Start pulling the gravel out. Use a cup or something to scoop it into another orange bucket. If you plan on keeping it and putting some back in, go ahead and drop it in a bucket with a small amount of water to really rinse it good. Otherwise, you can get it in a dry bucket and either trash it, or sell it on craigslist or something.
Next, once the tank is empty of rock and most, if not all of your water, start putting the sand in there. Easy! When you start putting water back in the tank, add it super slowly so it disturbes the sand as little as possible. Floating sand can mess up your filters. Use warm water from your sink or bathtub to make sure the water is between 70-80 degrees, otherwise your fish could go into shock from the temperature change.
Make sure you condition the water in each bucket before adding the water to the tank. (If you are open to suggestions, I use seachem Prime for my water conditioner and it works wonders! Though it is on the expensive side)
When the you have the tank a quarter of the way filled, add the decor back how you want it. You can always try a new configuration! Make sure any new driftwood or other things is rinsed and soaked before hand. Fill up the tank the rest of the way. If you added the water carefully enough, there should be some clouding, but not too much. Your filter should make quick work of it. Additionally, the water should still be warm, hopefully the same temperature as the tank was previously. Check the pH of the new water in the tank. It should be the same as as the pH in the buckets. As long as the pH is the same, and the temperature is very similar, put the heater(s) back in the tank and add the fish back in.
If the temperature or pH is very different, you'll need to acclimate them like any new fish.
Hope that helps!
