I think with the shells there is something to do with calcium leaking into the water form the shells. someone else may be able to shed a better light on that
ill take the shells out tonight there no big deal anyway im planning on getting some more plants that will hopefully root in as the plecs have eaten my other plants
If the plecs are of the same species, it would probably be better to separate them. Most of the species commonly sold (common plecs, gibbies, etc) can be territorial with their own kind. A common plec will probably need a 5ft x 18" tank at least in the long run. I have my 8" gibby in a 6ft x 2ft x 2ft tank and he already looks a bit big for it.
they get on very well togeather always lyeing next to each other no fighting at all.
i removed the shells last night and re-arranged my tank an bought 3 more plants but i found that one of my apple snails had died it had been stuck / caught in the filter