To put it plainly - I am an aquarist on a budget. This has led me to search for low cost alternatives to the overpriced aquarium gravel in stores. After searching a while for a solution that is safe for the fish and for the equipment, I have found a few viable options. One is suitable as a sand substrate the other is suitable as a gravel substrate.
I have read many articles online about different sands suitable for use in aquariums. The common recommendations are for play sand or sandblasting sand. I have found that the particles in play sand are too small and have very sharp edges. The small particles get sucked into filtration systems and can completely destroy pumps in hours or days. Sandblasting sand can be purchased in different grades that would allow for a large enough grain to ensure it would not be sucked into a filter. The main problem with sandblasting sand is cost - 50 pounds can be around $100!
The best solution I have found is to use pool filter sand. This is actually silica sand that is dense enough to not float in the water, and clean enough to not need a terribly thorough rinsing before use. The other benefit of pool filter sand is that the grain is smooth and round. Even if it does get into your filtration system, it is not nearly as destructive as other types of sand. Pool filter sand is widely available and at a cost of around $9.00 for a 50 pound bag you can't go wrong. The best part of pool filter sand is the final appearance. It is a light colored sand with speckles of darker particles. It looks very natural and the fish seem to love it. The other advantage to using sand is that waste cannot get stuck in it. I just vacuum a little waste off the surface and the substrate is clear of nitrite producing waste. Obviously, sand type substrates are not suitable for use with under gravel filters.
In my African Cichlid tank, I used an 80/20 mix of pea gravel and river pebbles to create a rocky, natural looking substrate. The pea gravel and river pebbles are around $3.50 for a 50 pound bag, so it is very affordable. It is important to check the pea gravel for alkalinity before using it in a aquarium. The pea gravel I used actually raised my tank pH about 0.4 after 3 weeks of use which was perfect for my African cichlids. To test the pH changing properties of your gravel, I suggest putting the gravel in a 5 gallon bucket and filling it with water. Test the pH of the water before adding it to the gravel, then test the pH of the water after being exposed to the pea gravel for several days. If the pH is still in acceptable range for your fish and has not changed by more than ±0.5 it should be safe to use.