This is an automatic fish feeder that uses a tumbler. A slot inside the tumbler can set the amount of food dispersed for one rotation. It can be used with flake or granulated food. For this project you will need:

• the printed parts
• an Arduino Nano
• the DS3231 Real Time Clock for reading the time
• the 28BYJ-48 5V stepper motor with ULN2003 driver board
• a push button
• a 10k resistor
• a 608 bearing
• 6 M3 nut inserts
• 5 M3x5mm for mounting the motor and the shaft
• 4 M3x10mm screws for the case
• 3 M3 nuts
• 6 M2x4mm self tapping screws
• 23mm of 1.5mm2 copper wire for the door hinge
• M3 screw and lock nut for optional mount

The tolerances for the parts are very small so that the gears don't rattle. I recommend using Prusa Slic3r with Ensure vertical shell thickness checked. All the parts print without supports except for the door and the dose setter which need some supports and a raft (optional).
Mounting:
Use the copper wire to connect the door to the tumbler. The door should open and close freely. Insert the dose setter in the tumbler. Connect the tumbler shaft to the tumbler with 3 M3x5mm screws and nuts. Press fit the bearing inside the motor case and mount the two nut inserts. Mount the shaft through the bearing and insert the big gear on the shaft. Insert the small gear on the motor shaft and mount the motor with 2 M3x5mm screws. Mount the electronics in the electronics cases with 6 M2x4mm screws (4 for the ULN2003 and 2 for DS3231) and with hot glue for the arduino and push button (insert the printed button in the hole before gluing). After making all of the connections between the boards, mount the 4 nut inserts on the back case and close it with 4 M3x10mm screws. Care should be taken so that the wires do not block the printed gears. For mounting on the aquarium there is a hexagonal hole on the bottom. I used the printed hexagonal peg mounted with a M3 screw through a drilled 3 mm hole in the top lid of the aquarium and a lock nut. The idea is that you need to remove the fish feeder easily in order to top up the tumbler with food. Of course you can design you own type of mount.
All the connections between the electronic components are described in the Arduino code. The stepper has 32 steps per full rotation and the gear ratio inside the motor is 64:1. The printed gears have a ratio of 3:1, therefore the tumbler rotates slowly (4 rpm) which is enough time for the food to enter the dosing part. From the arduino code you can set the time at which the tumbler rotates. You can multiply the current setting for more rotations.