I mix Easylife Catappa extract with my micro mix - it helps acidify the mix the keep the micros chelated, and means I dose it whenever I dose my micros. You just need to calculate the amount to add to the micro mix so that your weekly micro dose results in dosing your chosen amount of Catappa extract.
I would add it in one go, because instruction doesn't mention spreading the dose over the week. For example some fish medicines are meant to be spread across few days and then instruction is very clear about it.
But I wouldn't be adding weekly 8 ml per 100 liter just like that. I think it does not evaporate so I'd be adding only to compensate for the amount of water changed. If I'd change 25 liters of water I'd add 2ml of the liquid catappa.