Perfect timing, my two week vacation is 6 weeks away. Been trying to get my dosing routine fine tuned and estimate evaporation rates. With an open top, by 90 gal. acrylic loses about 1 gal. every 18 hours (I have an IceCap fan blowing across the top) which comes to about 14 gallons for the duration of my vacation. That's a bit much. Then I remembered that on my last vacation I just covered the main openings with the lids that came with the tank and evaporation was minimal (about 1") even with the back slot open. If you're still interested in an auto top-off, this place has a good selection including Kent's drip doser:
http://www.aquadirect.com/store/cust...me.php?cat=136
Drip doser rates will change over time. Calibrated drip rates when the container is full will be different when near empty due to a change in head pressure caused by the change if the volume of fluid itself. This might be neglegible, I don't know, but I plan on using a 1400 ml drip Aquadoser for all my ferts. I'll also be covering the container and using black tubing as a sleeve around the drip tube to protect the micronutrients from llight.
I don't expect much evaporation with my top covered so I'll save my money and not purchase any top off devices and only spend $18 on the small Aquadose. I just need to work out how concentrated to make the mix with all ferts and the drip rate. I gotta do this AND my taxes before 4/15, the start of my vay-kay!