Setting up 300 gallon Rubbermaid stock tank with underlayment and pond liner

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2bfishy2

Feeder Fish
MFK Member
Jul 10, 2025
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Hi, it has been a long time since I have posted on here. I was 2bfishy. But I couldn't remember my email address that I used for my account, so I had to create a new account with a different email address. So now I am 2bfishy2. I had a 8 foot diameter index snap set 500 gallon pool pond that housed mainly 9 sun catfish, 2 four line catfish, and various cichlids and buenas aires tetras, swordtails, and 2 oscars and a black spotted eel. I ran 3 tetra pond flat box filters totaling 1500 gallons of filtration and it was a very successful pond that I had up for 5 years.

Unfortunately, some repairs needed to be done in my basement and the pool pond was taking up a lot of space in the basement, so the repairman didn't have access to where he needed to repair, and I had to take the pool pond down. I took the fish to my local fish store and have been in a major funk ever since. That pond was my happy place and I really miss it. Fast forward a month later and I have gone to that same fish store every weekend for the last 4 weeks. This last week I saw they still have four of my adult sun catfish and 1 four line catfish left. They had them all housed in a 75 gallon tank. I feel really bad for them because they were in my 500 gallon pool pond their whole lives until now. I was determined to get them out of there. So I bought a Rubbermaid 300 gallon stock tank and put it in my basement on an insulated foam pad. I have read on various forums that these stock tanks tend to get cracks and leak so I didn't want to risk it and have bought a combination underlayment and 45 mil EPDM pond liner off of Amazon that arrived today.

Unfortunately, I have no experience with pond liners, only pool ponds, so I need some guidance on how this is done.

Do I just lay down the underlayment in the 300 gallon stock tank and lay the pond liner over it? I will be heating the tank with a room heater. Should I put any kind of insulation under the underlayment first to insulate it better in case of power outages?
 
Also, since I tore down the pond a month ago, I will have to do a new cycle on this tank. I have a 40 gallon community fish aquarium that has two seasoned 125 gallon sponge filters for filtration. If I put one of those seasoned sponge filters in the 300 gallon stock tank will that cycle the tank faster? Will adding Seachem Stability or Seachem Safe help speed up the cycle at all?
 
For the insulation part, that depends on the temperature of the room. If it gets cold then certainly I would put 2” polystyrene sheet under the rubbermaid and then yes to the filter, adding that will certainly speed up seeding and cycling the pond. Adding the liner will certainly help in case of any cracks or splits although I have no experience of using rubbermaid. Not sure you would need the underlay although if the room does get cold it cannot harm.
 
I set the stock tank on foam and didn't use the underlayment, only put in the 45 mil pond liner. I know you think its overkill but if you see there are several forums that are reporting that these Rubbermaid 300 gallon stock tanks are no longer built with quality, numerous people are saying that these tanks are getting cracks and leaking within 6 months to a year. I looked over the tank I bought from Tractor Supply and some of the seams looked sketchy. I read a lot of of people will have leaks and try to repair with silicone, JB Weld, etc. these tanks are not built as well as they used to be. So I figured it is better to err on the side of caution. I don't want to risk this happening to me, so putting in a pond liner ensures this tank will last for many years to come.
 
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Tomorrow I am going to fill up the tank with water and add one of my seasoned 125 gallon sponge filters and a 500 gallon Tetra Pond Flat Box filter. I really want to do a bog filter, but I am not handy have no idea you guys can explain it me and I still can't figure out how to build a bog filter. So I am just going to use the Tetra Pond Flatbox filters since they are easy to use and served me well with my 500 gallon pool pond that I had up for 5 years.
 
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I want to show photos as I continue with this build but I can't figure out how to post photos from my laptop on here. I know I was able to post some photos when I was 2bfishy but I can't figure out how to do them on this new Monter Fishkeepers website ??? now that I am 2bfishy2. How do you post photos on here?
 
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