Drainable Floating Fish Tank

Development and Testing of a Drainable Floating Fish Tank for Fish Culture in Deep Water Bodies

Project Information

Description of operation:

Years ago, after an unsuccessful day of fishing at my family farm pond, I came to the conclusion there must be an easier way to obtain fresh food fish. I decided to build a new pond to raise and manage my own fish. After formulating this goal, I met with several Iowa DNR officers to talk about my plan. After speaking with them about how I wished to manage my ponds, they informed me that I would be unable to accomplish my goal without a hatchery license. After obtaining my hatchery license, I formed Jim Frey’s Fish Hatchery in 1974.

Initially, this plan started out as a hobby, but I soon found out that almost every other pond in my area had the same conditions as my ponds. Upon discovering this, I formed a new goal: to help establish better fishing at my home in West Union and across Northeast Iowa. Over the course of 47 years, I taught myself through experience and by reading almost every article I was able to get my hands on. After many years of hard work, my business has expanded to Iowa, Minnesota, Wisconsin, and Illinois.


In the state of Iowa there are about 9,000 acres of abandoned quarry pits and 12,000 acres of abandoned coal mines, respectively.  Depending on the location, many of these have substantial water resources that could support aquaculture but are not being used due to their depths (bodies cannot be drained), which makes fish harvest difficult or nearly impossible. The goal of this project is to design a floating 7,500-gallon tank with 6,000 gallons of water available to hold fish.  Once the fish are raised to a desired size, a discharge valve will be opened (using no pumps or energy), and water will leave the tank allowing fish to be easily harvested.  This design was developed upon a smaller, earlier prototype. In addition to the energy savings, this tank will allow fish to be harvested from these systems with minimal equipment and effort. Upon the successful completion of this project, aquaculturists will be able to produce fish economically by accessing abandoned water bodies that to date have not been used for fish production. The limited amount of energy in the tank design will allow fish to be produced with minimal environmental impacts.

Project Objectives:
  1. Develop and test a floating fish tank for use in Iowa’s abandoned water bodies.
  2. Evaluate the costs and benefits of the floating fish tank for regional fish production.
  3. Share findings through field days and web site.


Materials and methods:

The floating tank has a 16-ft. diameter and a height of 5 ft. The tank was purchased used, estimated value of about $3,000 being provided by my fish farm. The design will have a 8-inch drain pipe with an exterior stand pipe which sets the water level inside the tank.  There will be four 4-inch air lift pumps which will put 240 gpm into the tank, which will create circulation and aeration.  This ensures a complete water exchange every 27 minutes.  The water will exit through a discharge drain that will have a 3/16ths stainless steel grader to retain the fish. Water will then exit the tank into the main waterbody which will act as a bio-filter. Figure-1-Sare-Project

To fill the tank with fish, the tank will be floated to the fish flue, which is a 6-inch aluminum irrigation pipe above the water which slopes down into the fish tank. A fish trailer can hook up to the flue, and fish can then be emptied into the tank.

The tank can then be moved back to the work area where it will be anchored while fish are being cultured. When the fish are ready to be harvested, the tank can be easily drained as described above in order to make harvesting convenient.

The decision to use Hybrid Bluegills is based on my experiences in culturing them, their hardiness under fish culture conditions, and their marketability in the Midwest.  Fish will be stocked into the tank in April 2019 and harvested in October 2019. Fish will be harvested periodically depending on growth rates and sold for stocking into private ponds.  In 2020, the tank will be modified if needed to improve performance and a new set of fish will be stocked in April 2020 and harvested in October 2020.

Research results and discussion:

Object of tank
Object is to float a 7500 gallon tank on quarry, fill with 6000 gallons of water, leaving one foot of freeboard, and then raise fish for stocking.  Use air-lift pumps to exchange water and aerate tank.  When ready to harvest tank, merely open valve and tank will rise out of the water and discharge all 6000 gallons without any pumps or any other mechanical device.

These things are true
You must know the weight of your tank, number of gallons to be used, weight of floor and additional rigging, floating capacity of your floatation per one square foot by one inch.

Small prototypes
Do all experimentation and calculations with a small prototype first.  A bottom of a five gallon bucket  works well.  It is one foot diameter (it is one sixteenth of a sixteen foot diameter tank).  All calculations can be made from this premise.


Tank leaks – very important
The statement, “ it leaks so bad it won’t sink”, is a nightmare.  Therefore, special care should be taken at each step, as if you were trying to keep it from leaking if set on the ground.  My major difficulties were under estimating the necessity of the tank not leaking. 

Cement collar should be at least 1 ft. on all sides and bottom of drain pipe. Silicone on top of cement before installing floor.

Floor is ¾ inch treated plywood and all framing is treated lumber.

A keel was considered for ballast, but with the use of prototypes it was determined it would take 12 foot deep and 5,062.5 lbs. of cement. This would interfere with the movability of the tank, because without it, the tank can be floated in 2 ft. of water when empty and 6 ft. when full.

In order to calculate the amount of floatation needed, you must first know how much weight a piece of floatation 1’ x 1’ x 1” will float, how much your tank weighs, calculate the weight of your floor and rigging, and know how much ballast you’re going to put in the tank. That will give you the amount needed for floatation, but nothing for discharging the water and refloating the tank that must be added into the amount needed. I believe if you were willing to pump the water out, the system could be streamlined somewhat.

The tank can be completely constructed prior to floating it in your water, but cement floor should be only installed after floating tank.

Cement floor is 1.5 yards, for ballast ( 6075 lbs. ) It has a ½” slope toward the center drain.

The original plan was to fiberglass the seams on the tank and the seams on the floor. However, cementing the floor and epoxying the seams was less expensive and worked better.

It was originally planned to have the framework, and sunscreen framing separate from the tank. However, having it attached made it easier to construct.

Once in the water, we filled the tank and proceeded to balance.

The water on the inside of the tank is 14 ¾ inches higher than the water in the pond. Without this difference, the tank would never rise out of the water on its own. It was necessary for stabilization to install outriggers and a walkway with additional floatation at the top to compensate when workmen are on the walkway.

There are a few minor adjustments needed which will be done in the spring before tank is refloated. The tank is on dry dock for the winter.