@oscar @Olpea thank you for the detailed explanations and analysis of my options! I understood a lot more about the motor, pump, VFD and VARIAC now. Thank you for being so patient and explain them so well! It seems buying a new pump will be much better option than buying a VFD or VARIAC. I am going to experiment with putting the pump remote as suggested by Mike and see if it will reduce the noise, then decide what to do next.
@PatapscoMike Thank you for suggesting putting the pump at a remote place and on a timer! What a great idea! Per your suggestion, I just tried to put the pump in the garage, itâs still too loud. I can hear it in the room above even during the day. Now I am going to try the walk-out basement. But I know I have to consider air valve because basement is lower than the pond.
So you are a fisheries biologist! I am just amazed by the diversity of the expertise on this forum. I have a question about the dissolved oxygen level, are there good and reliable ways to measure dissolved oxygen level without spending a fortune? I have tried to measure the oxygen level with a chemical set(Salifert Oxygen test), it couldnât detect the difference before and after the pump is turn on for several hours. It shows both at around 6(mg/L)(air temp was 90F) I thought maybe the set was bad, but it did show the level to be at 8 when it is cooler (air temp at 55F and 70F)
I looked up digital meter on Amazon, it starts at $100, and goes up quickly.
I may have more questions later on.
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I actually thought about using timer before Mike suggested, it never occur to me it might be dangerous. 
@clarkinks wow, you are an expert in raising pet fish! Do you have any suggestions about best way to measure water oxygen level?
To answer your question:
Yes, I use multiple layers of substitute pond filter medium. When we first set up the pond, two pressurized filters and UV light were used, the water got murky often and we had to wash the filter all the time. Then, one day I came across âskippy filterâ online, now I just use my own modified skippy filter. Basically, I feed the pond water to a 80-100 gallon upper pond, water goes through the multiple layers of filter medium in the upper pond, flows down the creek and back to the pond. I then grow watercress and water spinach on top of the upper pond. They used up a lot of nutrients in the water and their roots also help trap a lot of the sludge. Once or twice a season, I use shop vac vacuum up the sludge. By the end of season, I dissemble and clean the biofilter Aka upper pond. I also have a lot of water Lilly in the big pond. It was a very low maintenance operation before the arrival of these koi , and it also provides me with unlimited supply of watercress 
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Not that I am aware of. Itâs expensive to measure. You can manage your dissolved oxygen in many ways, but the key is to have a reasonable number of fish for the size of the pond and to minimize fish feeding. Need to keep nutrients out of the water as much as possible.
Good luck!
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@Sara_in_philly
Experts that raised fish refer to me as an expert on fish. Many people asked me to write a book but i declined. Many times trying to explain to someone your system is not going to work out its a very advanced system. Itâs not about power itâs about your entire system which is very high end. Once some friends owned large oscars for years, a male and female and asked me why the fish never bred i said i dont know maybe they dont like each other. When they showed me the fish i said all cichlids need a cave or they dig a pit to reproduce. A catfish couple does the same thing a hollow log or pipe. The fish and ponds get bigger but the principles are the same. So i placed a 5 gallon bucket in there and the next day the oscars laid eggs inside the bucket. I help people to get their ponds straightened out as well. Same with angel fish people had trouble because noone added black water extract to their water which they need. They are from the amazon and they want the right water quality and a stick or something going vertically to lay eggs on. I use METHYLENE BLUE for fish eggs once the eggs are laid to prevent fungus. There are multiple ways to test oxygen a kit like this https://m.youtube.com/watch?v=9Jk0AmmuLSY or an oxygen meter which is not cheap. You get down to it most fish experts would refer to you as an expert based on what your doing. If people knew the cost of koi or the work they would be shocked. One fish can cost thousands of dollars. Was your friend originally from Japan the koi are very popular there? They are some of the best in the world. Koi in Japan are like race horses are to Kentucky. Here is an example of koi for those who may not know Joe's Fish Matsu Kawabake Standard 21 Female [BMG60-622020-045] - $1,699.00 : Black Water Koi Farms!, The Best Koi In The USA
@clarkinks I know some fancy koi can cost a fortune, but mine are not that kind. They are just common koi. Their previous owner is moving to a new place that doesnât have a pond, and want to find good home for them. So I and another person each took half of them, a water pump and a air pump came with the fish. If they were the fancy ones, I am sure the owner wouldnât give them away for free.
These koi are fairly big, ranging from 1 to 1.5 ft, they feel more like pets. I just want to make sure they are ok.
Now I know you are an expert, I know who to ask if I have questions.
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Just want to give everybody an update. I have tried to put the big air pump to the garage and basement, itâs still too loud. So I decided to try new pumps.
The first air pump is a Hydrofarm AAPA45L Active Aqua Commercial, 6 Outlets, 20W, 45 L/min ď˝$33. It states that noise level < 45db. Although quite a few reviewers complain about it loud, but it was very quite for me. When using outdoor, I canât hear anything from the pump during the day. But the power is a little weak, acceptable, but weaker than I like.
So I ordered another one for almost the same price (50cents difference) VIVOHOME Electromagnetic 32W, 60L/min. The noise was < 65 dB. I thought the 20 db difference will make this one really loud, but to my surprise the noise level is almost the same as the smaller one. And it pumps plenty of air. So I decided to keep this powerful one.
Now, I just need to find a timer for it.
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