Temperature has a big effect on maturation rates. Even if two tanks are unheated, one will often be a few degrees warmer due to lights, heat from powerheads, etc. Worth checking into, but it doesn't seem to be the only factor. Guppy fry almost always mature faster in my 46G fed once daily, which is my largest and runs about 77°F average on its own; than any other tank deliberately set up for fast maturation, heated to 82°F, and with feedings three times per day. Strange that tank size, scavenged food, and perhaps other unknown factors, seem to overwhelm more direct influences. For a tiny fry, even a 10G is a vast space.
And as for fish learning, it's all cool until one decides to start eating live plants... and others start mimicking it too! I had that happen, fortunately just once, and had to get rid of a few fish to shut it down.
Aquariums are always an adventure.
If you do end up changing the substrate and later find that fry mature faster, let me know.