- A plastic container, clear. I used a "dry-goods" container, but any container that holds liquid and is large enough will work.
- A powerhead or pump. This should be small enough to fit inside the container you've got but also strong enough to pump liquid into your tank (especially if you will be storing the fertilizer tank under the aquarium as I did). It also should have an intake on the bottom.
- Tubing to fit your powerhead outlet. Only a small length is needed, a few inches max.
- Airline tubing.
- A plastic dropper (as found in Seachem liquid fertilizers).
- A digital timer, or any timer with the ability to be set to 1 minute intervals.
Then, hook everything up (airline tubing to dropper-adapter), place the powerhead inside the plastic container and fill it with plain, de-chlorinated water. Set it up exactly as you would if you had put fertilizers in it. Make sure the airline tubing opening sits above the water line in your tank or you may unintentionally start a siphon and have a big mess! Alternatively, you can use an in-line check valve, however, my powerhead wasn't powerful enough to push the fluid past it and all the way up to the top of the tank.
Most timers have a manual override which we'll now use to simulate a minute run. Plug your powerhead into the timer. Just watch a clock and run the pump for a minute on and then turn it off. Mark the side of the container with a line. This indicates 1 dose. Keep doing this until the container is empty or nearly empty. This is the most tedious step and requires the most attention, since the more accurate your measurements are, the more accurate your dosing will be.
Now count how many doses you have and this is how many days you can run your auto doser before having to refill it. Mine ended up being about 28 days worth. Then, just use this number of days to calculate how much fertilizers you would dose in the same period, and add it to the container. Do not add phosphorous to the mixture if you are adding any type of iron fertlizer! This is because they will react. So I just dose nitrogen, potassium, trace elements (including iron), and Flourish Excel in the auto doser, and phosphorous separately. Add de-chlorinated water, mix it up a bit, and hook everything up again and you're good to go! Set your timer "on" time for 1 minute before the "off" time. Doesn't matter what time of day, though most people will tell you that right before "lights on" is best. Every day, the timer will turn on for 1 minute and dose 1 days worth of fertilizers to the tank. Just remember to check on it now and then to make sure everything is working as intended.
This is probably the best DIY project I've ever done in terms of impact. Makes keeping a high tech aquarium so much easier!