Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Put some of the gravel in a glass of water.
Test GH, KH, pH and TDS of the plain water, then test it in the glass with gravel every few days for a week or two.
If the gravel is of limestone origin then it will add minerals to the water. This may be OK for certain fish (live bearers, certain Cichlids, certain Rainbows), and the plants will not generally care, these minerals are some of the trace nutrients they use.
But if the gravel is too active this way you will not be able to keep most of the soft water fish that are so often the choice for a planted tank.