Yellow wiithering older leaves

I have been growing my plants indoors for about 2 months now. At first I had problems with fungus gnats because I bought cheap soil from WalMart. So I took a friend's advice and added about an inch and a half of sand and it helped with the fungus gnats but after I transplanted my plant the oldest leaves on my plant are turning yellow and withering.. I'm scared that my plant is gonna die! Help!

bump...i am curious too. I have healthy looking plants, but they have had some heat issues ~82-84 the last 5 days, but the portable A/C finally found its way from "lost" to delivered! They look great on their tops, with a little slowed growth from the heat, but most their lower leaves have died off or are very yellow.