150 gallon pond - SELA - I want a fish

2 or 3 koi are a good option, and maybe 12 or so mosqutofish (Gambusia affinis).

Filtration, just google filtration units for small koi ponds, plenty options, for less than $100 and find an option you think might work best for you. Options include filtration units submerged in the pond, and those outside the pond, with a submersible pump that pumps water into the external filter - making it logistically easier to clean. You can disguise/hide an external filter with plants, etc.

I doubt you need a heater but small submersible external heater can be purchased if needed down the road in temperature extremes.

Obviously if the unit has a fountain you have power source for a filtration unit.

Many people with ornamental fish ponds have a tendency of over feed the fish. Seldom should fish be feed more than 3% of body weight per day (growth diet) and a maintenance diet (non-growth) is about 1%. Floating feeds are best because see the fish when feeding to check on their general condition. There are many “natural” foods in the pond that the fish will eat.

Here is an LSU AgCenter publication on the subject to get you started. Enjoying Ornamental Fish Ponds in Louisiana

I had a pump run 4 hours each night and kept 5 goldfish in a pond half that size. Did cleanings every three months (scrape out leaves and water change, add prime water treatment) and the fish lasted for 4-5 years. Even through freezes (although, I would get a tank heater if you get below 30 for too long). I didn’t even feed them - they will eat algae, tad poles, mosquitoes.

Ribbon snakes and birds will try to get them, so give them lili pads as cover. Also, shade the pond as you need the right amount of algae. No direct sun.

I got the fish as “feeder fish” from petco. They turned into beautiful, long tailed gold fish about 5” long.

Blue John Jody done got yo fish.

Install a UV light is a must. That kills off some of the green slime. Beef up the filter system as much as you can afford.

150 gallon is small for koi as they get bigger. Lots of poop. Very easy to over fish your pond.

Koi do not need a heater. Koi will survive in a pond with a frozen surface. They sit deep and don’t move. Tropical fish usually die in winter. Prolonged temps under 55.

Do not feed koi in temps even nearing 55 degrees. Their metabolism slows so much that food just sits in their belly and makes them sick.