DIY – Monster Indoor Fish Pond

Today we’re posting a DIY article on how to construct a Monster Indoor Fish Pond. This one was built in a small garage but you could put one anywhere you want, just remember you may need a dehumidifier if you don’t have lids or good ventilation.

You can adjust the design and size needed to fit your location as well, using a liner makes it easy to modify because you can make it any shape you want. This specific pond measured out to be 8′ long by 4′ 7″ wide external dimensions, the inside dimensions were about 7′ 2″ by 4′. It does narrow down to about 3′ wide in the rear but you could make it a normal square as well. This ends up holding around 400 gallons of water, depending how high I filled it up. Anything more than 30″ and it is hard to each the bottom, keep that in mind.

Approximate Materials needed for this pond were as follows.

  • 15′ x15′ Firestone 45mil EPDM Liner
  • 30-35 8′ 2×4″
  • 15 sheets of 2’x4′ 1/2″ Plywood, you could use bigger sheets as well
  • 4 sheets of rigid insulation 4′ x 8′ sheets
  • Expanding Foam (Optional, I used it to seal some foam insulation gaps and to hold the bottom sheets of foam together then covered it all in tape)
  • A few hundred 2.5″ and 3″ screws
  • Metal L Braces for the corners (could probably go without but I am careful)
  • A couple rolls of duct tape to secure the insulation


Just a rough setup to see what kind of room I had.

Frame built, still needed more bracing. This is just 2″ x 4″ screwed with 3″ screws.

Added more vertical braces, looking good now.

Time to mount all the Plywood, this is held with the 2.5″ screws. I also added a bottom 2″ x 4″ brace to keep it from bowing. Nothing is needed on the floor for support, concrete is pretty strong. We will add some foam insulation to the floor though.

Notice the metal braces on the corners. I did these on all the internal corners. Just for peace of mind.

Adding the rigid foam insulation. Not only does it insulate the pond but it also protects the liner from punctures. It is nice and smooth, no risk of the wood cutting into it this way. The fish can also bang rocks around without worry because the foam has a little bit of give, not much but enough to help prevent accidental punctures.

Adding the expanding foam to the joints. I thought it would make the bottom more realistic instead of being completely flat.

Covered it all in tape to hold it all in place and to give the liner a smoother surface. The expanding foam can cure a bit rough.

Putting the 15′ x 15′ EPDM 45 mil Firestone liner in. Once in you can add a bit of water to help you get creases out. The weight of the water makes it easier.

Added a top brace to help it from bowing. About 2′ of water at this point, which is roughly 360 gallons.

Here the brace was covered in liner material and the pond was filled up a bit more to about 400 gallons. I also added the waterfall which also serves as the filter. I just built it out of scrap leftover wood and liner, nothing to complex. The liner was also trimmed and stapled down on the edges.

Here is the pond with tiles on the edges to give it a cleaner look. I also added some driftwood, art work and plants. The waterfall provides filtration, oxygenation and water movement.

Here are some fish enjoying the new digs. An Oscar cichlid, Venezuelan Pike cichlid, and a Niger Catfish. You can also see a Bichir (Polypterus) hanging out on the left side.

You can see some Bichirs, Jaguar Cichlid, Oscar, Pike Cichlid and a Convict.

One last shot of the mega pond.

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