There is lots of info on rain barrels. We have 2 on each downspout. To access the water, I cut off a section of the lid with a saw and scrapped the edges with a deburring tool. A lot of plastic got into the rain barrel, I scooped out as much as I could.
The goal is to circulate the water between two rain barrels, to give the fish 100 gallons of water. Fish can breath under water (of course) but they still need air to be in the water. Small fish tanks often have an air pump, but air can also be added by making a little water fall. In this case the water cascades from one barrel to the next.
This shows the basic setup, water is pumped from the lower barrel into the upper barrel. I've removed an overflow spout to allow the water to freely flow into the lower barrell.
If the pump is faster than the drain, the upper barrell will overflow and you will run out of water. If water spalshed out of the bucket, you will run out of water.
To solve these problems, I added a clamp to slow the flow from the pump. I do not know what this clamp is actually for. I also added a pipe to keep water from dribbling down the side of the barrel. Wrapping the pipe with electrical tape, them jamming it in a hole, makes a workable seal.
Step 8: Add Fish
Let that run for a while to make sure there are no leaks. Then dump in the fish.
After a few hours, the open barrel started attracting insects which the fish can eat.
Step 10: Maintain the System
At this time, we have been running the system for over 2 weeks. The 5 fish are still alive. When it rains we drain off water into the garden. The pump has needed to be wiped clean a few times. We will have to add a coarse pre-filter, like a bucket with holes.
I have to add a section of plants to filter the water.
The next step is a fish autopsy. It shows why one fish with a stuck hook died.
I have noticed that sometimes when you catch a fish it will suddenly die. Other times it might still be moving after you've chopped the head off. I carefully cleaned (cut open) the fish that died. It turns out that the hook was directly next to the heart.