It is worth it. Sand is so much better. The poo does stay on the surface. As long as you have good water flow in the tank, which a Fahaka would probably like, the flow will push the poo under any decor you have in the tank, hiding it until you vacuum.
Get a heavier sand. As mentioned, playsand is not good. Get pool filter sand or 3M colorquartz. Keep the Fahaka in the tank when you change to sand. Empty the water 1/2 or 2/3 of the way. Use a cup to get all the gravel out. Then use a cup to put the sand in. Even if you rinse sand very well, it still tends to make the water cloudy for a couple of hours, but it will clear. 100 lbs of sand will be more than enough for a 125. I think I have 60-70 lbs in my 125.