[vijaymcud]

This is regarding some aquarium hobby. I have tried much but have given up on aquarium forums to try the knowledge of chemists on this question.

The total dissolved solids (TDS) must not change between the source and when it is added to the substrate (clay pellets). In other words the substrate should be inert. I brought the clay pellets because my local fish store vouches for it for plant growth. They talk a lot about Cation Exchange Capacity (CEC), the ability of the plant to root properly etc. I do know the benefits of such a substrate for plants.

But the problem is this. The minute I add the source water (250 PPM TDS) to the clay pellets it becomes 850 PPM TDS. This is not good for the fish, the plants etc. The term for such a water is "liquid rock" within the hobby. Now for the questions,

What exactly does my burnt red clay pellets leach into the water? (Calcium, Iron?)
With this eventually be dissolved and washed away over time or will it keep leaching?

[Corribus]

Most clays are layered aluminosilicates, surface functionalized (site swapped) with other common cations (iron, magnesium). They can also have a lot of adsorbed organics (surfactants and otherwise). Without more information on the clay, it's hard to say specifically what could be dissolving off of them, but my off-the-cuff guess would be some combination of organic surfactant/stabilizers, iron or magnesium salts, or dissolved alumina/silica.

[billnotgatez]

from my experience with pottery
red clay contains iron oxide
I do not know about aquarium clay

[vijaymcud]

Thank you Corribus and billnotgatez for your inputs.