Planted Tank Guru
Join Date: Jun 2004
Location: Baton Rouge, LA
No, you will not see a measurable pH change at all, as glutaraldehyde does not release CO2 in the water.
Glutaraldehyde is actually a toxin. Which occurs naturally as a by-product of various metabolic processes, so all living things in your aquarium have protective mechanisms against reasonable amounts of it.
Once glutaraldehyde enters any cell, these mechanisms immediately start breaking it down to render it harmless. Plants break it down into a form carbon they can use. Whether it's actually CO2, or some other form of carbon, is unknown. But as this occurs only within cells, the carbon is produced and used there, and never released into the water in significant amounts. So no pH change.
Glutaraldehyde is only shelf stable in an acidic solution. A bit of acid is added to Excel, Metricide, and so on, so that it doesn't expire quickly. This is why when premixing a diluted solution of any of these products, only distilled water should be used; if high-pH tapwater were used instead, the glutaraldehyde would start breaking down immediately, and lose most of its potency in a few weeks. A few people have made the mistake of adding Excel until they see a pH change, expecting it to be caused by CO2. But this is due only to the acid component, and they've really added a massive and harmful overdose.