OK I found 90 lbs of black substrate on ebay. I really like the color and it would only cost like 25 U.S. dollars. I was wondering if 90 pounds (40.9 kg) will be enough for a 125 U.S. gallon tank? The base of the tank is 6 feet by 1.5 feet.

First I want to know if this is enough substrate, and second if this type of substrate is safe.

It is....

MATERIAL.............................. ACRYLIC COATED SAND GRANULES

COLOR......................................... JET-BLACK

ORIGIN.................................... SOUTHERN CALIFORNIA

CHEMICAL NAME....................... CALCIUM CARBONATE

PRODUCT USE............................ MINERAL ADDITIVE

SYNONYMS ....................... SAND, GRAVEL & CRUSHED AGGREGATES

If it is acrylic coated then it shouldn't alter the chemistry of your water. I would say that 90LB of substrate would be more than enough for your tank. Add the substrate gradually and when you have covered the bottom of the tank completely and come up around 1-1.5 inch, that would probably be enough in my opinion

Ok cool, well I think I'll buy it, does it seem like a good deal?

I bought 100lbs of sand and pea gravel for my 135gl, and it had a nice 1.5-2" layer on the bottom. So 90lbs should definitely give you enough substrate. If the sand is indeed acrylic coated I don't see any problems. And $25 is pretty good for black sand. It's usually much more. (That's why I hit up Home Depot and paid a total of $7.26 for 2 bags. I'm cheap, what can I say? lol)

my tank bottom is the same dimension (72" x 18") and I used 125 lbs of black gravel. It's 1 1/2 inches thick.

You can always buy another 25 lb bag of black gravel if you find its not thick enough for your liking......

Well I lost the bid for it so I didn't get it. but the 125 I just bought came with gravel so it's all good