All true but when you mix with sand and add water a chemical reaction will start and you end up with a mix of calcium sulfate, silicates , oxides, and calcium carbonate. and calcium hydroxide. Calcium hydroxide is lime and will push the PH up. Depending on the brand and the chemistry of the water yours you could get a lot of calcium hydroxide or very little.Lime or calcium oxide, CaO: from limestone, chalk, shells, shale or calcareous rock
Silica, SiO2: from sand, old bottles, clay or argillaceous rock
Alumina, Al2O3: from bauxite, recycled aluminum, clay
Iron, Fe2O3: from from clay, iron ore, scrap iron and fly ash
Gypsum, CaSO4.2H20: found together with limestone
It's the extruded insulation foam (pink, blue and green)...has no fire retardant chemicals in it. Nor does the expanded (white bubbles) foam.I personally would be worried about the chemicals in the foam, surely as a minimum there will be a fire retardant in there. that will leach into the tank over time
I once thought that calcium carbonate was not soluble but in fact it is. But it is less soluble thanCalcium hydroxide. Cement is not soluble because the calcium hydroxide reacts with sand (Silicon dioxide) forming calcium silicate which is not soluble.Now that I have a likely cause, there's something I think I can try. According to the wikipedia page on Calcium Hydroxide (slaked lime), Ca(OH)2 dissolves in water, raising the pH to up to 12.4. Ca(OH)2 may then react with CO2 to produce Calcium Carbonate. Calcium Carbonate is not soluble, unless it reacts with more CO2 to form the soluble Calcium Bicarbonate. As far as I can tell, Calcium Bicarbonate does not change pH and is not harmful to fish? In any case, you can remove it from the water by doing a water change.