Just incase anyone wondered, as Excel has the additional benefit of being an effective algaecide in some circumstances, normal dose does NOT kill moss balls.
I got a response from the Seachem tech support on this today.
Interesting. I use EasyCarbo at double dose and wondered if it was responsible for my moss balls apparently shrinking! so I moved them to another tank without liquid carbon to see if they pick up.
Good to know I use 2-3x the recommended dose and my three moss balls are doing well. Of course now that I said that you can expect me to report back about how they are now deteriorating.
I wouldn't use EasyCarbo at more than 2x dose. It states on the bottle that this is not recommended. I assume the same is true for Excel as they are supposedly similar.
The instructions for Excel are a little more vague (taken from the Seachem website)....
On initial use or after a major (> 40%) water change, use 1 capful (5 mL) for every 40 L (10 gallons*). Thereafter use 1 capful for every 200 L (50 gallons*) daily or every other day. Dosing may be slowly increased in high-growth aquariums. For smaller dosing please note that each cap thread is approximately 1 mL.
The increase dosing slowly for high-growth aquariums part is vague in my eyes as it doesn't provide any max. On the bottle it then goes on to state don't overdose. I should have clarified that I mostly dose 2x as from what I have read that is fairly typical. I will throw in a little extra usually after a water change or to combat some algae but it's not a usual occurrence.
The problem with that product is that if you are heavily planted you will require more or less the recommended amount. It is to be used as a general guideline for paranoid people such as me. It's impossible to write on the bottle all possible combinations.
The reason they say add slowly is because that way, you will be able to prevent a disaster in case you overdose.