I am trying to test/determine how much CO2 is in my 180Gallon fresh
water tank. It has been running about one year with a 20 gallon tank
of CO2 with needle valve, electronic shutoff when lights go on plus
I have another pump that comes on at night that ruffs up the water at
night to try to release any left over CO2.

Only once was I putting to much CO2 into the system, I was able to
determine this by looking at the fish and seeing most of them were up
on top trying to get a fresh breath of air. (CO2 was on during the
night) that was a X10 unit that missed its call to shut off. Its now
on a mechanical timer with no problems.

But trying to test to make sure I am putting enough into it, seems to
be miss leading. I am using a Salifert CO2 tester. 5ml of water, 6
drops of #1, turns the water yellow, then add drops from #2 bottle
until water turns blue, it takes 25 drops, which seems excessive and
if I am reading it correctly 1 drop of number 2 = 5mg/L, I read 15-20
mg/L is the normal range. If I am doing this correctly, would mine be
125 mg/L?

The fish and plants look fine. Am I doing the math wrong?

Thanks

James