7 seconds every 2 days? Just curious but did you read anything to suggest that it should be so infrequent or short? I mix mine for 1/2 hour a couple times a day.
What a profound difference. I use a diaphram metering pump set a rate which is slightly less than evaporation (and just use a float valve for the remaining topup). I found my evaporation rate using trial and error. Kept slowly increasing it every couple of days until I noticed the sump volume increase slightly over a 24 hour period and then backed it off to the last setting before that.