Will Carbon In My Filter Remove Plant Nutrients?

Carbon won't adsorb metal ions in the fertiliser as they form ionic bonds and carbon forms covalent bonds. But metal ions will combine with organic molecules in the water and the organic molecules will bond covalently to carbon, which will have the same effect as the ions adsorbing directly.

If you leave the carbon sponge in the tank for more than a few weeks, it will be completely saturated and shouldn't have any effect; it will act like a normal sponge and house bacteria. But having said that, I don't use carbon routinely. I do have a bag of carbon in the cupboard which I put in a small cheap internal if ever I need it.

The salts added to the trop mix will be the same that are found naturally in water sources, just less of them to make it softer. They may not contain the trace elements the plants need.

With the carbon sponge, carbon works by attaching things to its surface (adsorption rather than absorption). Once the surface is full, it stops adsorbing things. That's why you have to replace carbon every couple of weeks if there is a reason you need it. Carbon itself is not a terribly good home for bacteria, but yours is impregnated into a sponge so it will make quite a good home for bacteria. You can leave the carbon sponge in permanently, it will stop adsorbing things after a couple of weeks and just function as a biomedium.


The only problem could arise if you ever need to treat the fish for an illness. There is the risk that the medication might be more strongly attracted to the carbon than what's already on it, push the other stuff off and stick to the carbon instead - and med stuck to carbon doesn't cure fish. Most people don't have problems though.