bummer deal you might have an issue in that tank considering the crap from that lizard im not a reptile guy but im assuming its waste could be poisoning your fish btw white gravel does wash out a basses colors i like the more natural colors or black it makes the gold on bass explode
I'm with you on this. The waste from the lizard is probably killing the fish. It may be disease or could be the chemical makeup of the lizard excrements. The only way I can see this working is if the lizards was very small (small amount of waste produced) or the volume of water was large enough to dilute the toxic byproducts.
I'm not an expert but I'm guessing this is causing the deaths.