I'll second the sump suggestion if you are cash-strapped. The ready-made sumps are a bit pricey, but it's easy to make your own. There are lots of DIY sump threads in this subforum, so I'm sure you can find one suited for your tank and skill level. A bonus is that you can put the heater in the sump and have it out of the way.
If you're figuring out how much sand you want, first figure out how many cubic inches you need: tank width X tank length X desired sand depth (all in inches of course). A standard 50# bag of sand is 1/2 cubic foot, or 864 cubic inches. So, divide your first number by 864 to see how many bags you need. You said the light is 72", so I'm assuming the tank is too (probably a 125 in that case); standard dimensions for a 125 are 72" X 18" X 22"H. So, 72X18=1296/864=1.5. One and a half bags of sand per inch of sand depth. I'm guessing you'll want fairly deep sand for your ray.
If you do get an oscar and a ray, you've already started an Amazon biotope. Something to consider when picking other fish.
