Chia,

100ml + 400ml is 500ml. 100/500= 0.20 0.2x25%(original concentration)=5% solution of glut.

5% is that same as saying 5ml/100ml H20. Since gluteraldehyde is ~ the same weight (density) as water you can safely assume 5% is the same as 5g/100ml, or 50mg/ml

So, providing I have done the math right (not my strongest field), and assuming that a 20gallon is really 20gallons (75liters). Dosing 10ml of your solution is: 10ml x 50mg= 500mg/75l= 6.67mg/l!

For more on the math see post 46, 47, 52 & 53 of this thread. That's a pretty high dose! I experienced a high die-off of *Riccia* at half that. The MSDS of gluteraldehyde lists toxic levels as 1mg/l for some inverts, though what those inverts are remains a mystery.

In my experience, the DIY version seems to have some added benefit but its not quite as good as Excel. I read receintly what the concentration of Excel was, though I don't remember now. Maybe 2-2.5%, IIRC. So ml per ml, DIY vs. Excel, your concentration is about double.

Let us know how it goes.