hi just checkin out your thread. you are on a good track using natural stone BUT!!!! when and where you buy it please ask the seller if the ston is presealed as it could most definately hurt your fish. i am a career floor mechanic and work with this stuff all the time. now as for installing the stone i would sand set it, basicly put a thin layer of pool filter sand on the bare glass bottom,say 1/4 to 1/2 inch,then placing your stone in whatever layout you choose, myself i would leave small joints between peices 3/16 inch or so,then filling to just over the stone fill these joints with more sand. now this will allow a layer of sludge to grow beneath the stone but it may actually be beneficial nad if you decide one day to remove the stone it will be a lot less work.i deffinately DO NOT reccomend using grout or thinset as these both are cement based and have very high levels of lime and other bad things your fish would not like. there are a huge varieties of travertine , marble and granite but the price can be staggering. ceramics and porceleans will work but to me lack the carachter ,wow my spelling is off must be tired,anywho yes silicone would be a better choice as far as making the tiles more permanant, but allow much more drying time as these will not allow the silicone to breathe and dry as quickly,again the silicone mixed with pool filter sand would make a good grout. whatever you choose post pics would be interesed in seeing.GL
LFV