I can't answer the one about the oxygen but I can answer the one about the weeds. The wheels of the train produce sparks which will often cause brush fires where weeds are left to grow.

Just to be safe add an aerator line. Place the pump as far as possible from the tracks and cover the pump with a tub of some sort.

Contact the company spraying the weeds and advise them there is a pond and to please inform you as to their spray schedule so you can prepare for it. As for an aerator for the pond during spraying let me tell you about a story from our water quality expert.

One year at a koi show they had an issue with koi getting lethargic and not looking well. Turns out that the airpump was at ground level and was sucking in all the air at ground level and sending it thru the lines to the tanks, which it is supposed to do, but the air was filled with emissions from cars and such pulling in to drop off fish and equipment as we know CO2 is an issue. So every year now we have a snorkel that goes 12 feet into the air for insuring better cleaner air
What i am saying is that if you use an air pump please make sure it is in an area where it will not suck in the pesticides and pass it thru to the pond Maybe weed whack the area directly behind the pond so they don't spray it.

Here is a link that might be useful: koi club

That is a really good tip. Thank you.

Oxygen loss is also determined by the temperature of the water. The warmer the water the quicker it loses oxygen and thus harder to keep oxygen in the water when the pumps are turned off. I have my pond ran off 1 circuit and use an in-line light switch that I can flip to turn off the pumps when I need to do a little maintenance or sometimes I need to switch it when its very windy outside so I don't lose a lot of water from my fountain and spitter. My pond is only a 50 gal pond so oxygen loss would take quicker affect on the fish and I have already went 2 days with my pumps turned off and never had a problem. If you are worried about oxygen loss for the 1 day you have the pump turned off you can add some fresh water to the pond to give it some oxygen.

This link will lead you to an article by Norm Meck who is a leader on water quality. It discusses dissolved O2 and other chemicals. I would recommend that you read all the articles as they are very informative.

Here is a link that might be useful: Water Chemistry

This link will lead you to an article by Norm Meck who is a leader on water quality. It discusses dissolved O2 and other chemicals. I would recommend that you read all the articles as they are very informative.

Here is a link that might be useful: Water Chemistry

I apologize for the double post I tried to delete one
but couldn't figure it out

If you keep an aerator on, you will be fine. Also, the cover will keep the water temperature lower and thus, keep more oxygen in the water as Jason34 indicated. Do not let the tarp lay on the water, but keep it elevated as the oxygen exchange occurs on the surface.

I honestly wouldn't use a tarp. Actually I wouldn't use a cover at all. If the sun beats down on the cover it will heat up everything its covering. Obviously I would leave the cover lifted off the water like magnoliaponds said to allow air to run under it. If you do use a tarp go with as light of a color as possible.

I agree, Jason34. But if the poster's original concern was they are out there spraying Agent Orange (I am dating myself) then a light weight, light colored tarp is the best alternative. The heat is only one reason to keep it elevated a few inches, it allows heat to escape and the natural exchage if surface oxygen. The required use of an aerator will keep a flow of cooler air in and through the tent.

Not trying argue with anyone.

Got to watch when using a tarp though because they are woven thus liquid can still seep through it. If you can get ahold of an actual plastic cover youll be in business

Shalom - maybe a clear plastic paint drop cloth - cheap, easy to store or even throw away. fold the plastic over a couple of 1x2', staple and roll it up when done ?