Hi, I'm new to Koipen and new to keeping Koi.
I live in The Bahamas -- climate is HOT (and for anyone wondering...I live in Nassau where we were not affected physically by Hurriane Dorian)

My husband built a pond a couple of years ago and once the water had settled we added some koi that a neighbour brought us from his well stocked ponds (he breeds them). Our pond is about 1000 gallons is I can tell (7 feet long, 5 feet wide, 2.5-4+ feet deep; shallow at one end, slow decline slope and a little more than 4 feet deep at the end where we keep our filter). The Koi appear to be some "regular" or common koi and others butterfly koi.
I really wasn't interested in either the pond or the fish until a heron (great blue believe it or not, the things was huge) came and stole most of our fish around December/January last year and then I began taking notice. I thought all the fish were gone but slowly they came out from hiding behind the filter and it turned out there 4 were left. I had no idea if they were male or female. They swim around happily, were growing nicely and in the spring we had babies!! Hundreds. (lol..I know they are called fry but I'm a mom...they became my babies and now I care).

It appears the menacing heron left me with 1 male and 1 female each of both the common and butterfly variety. Unfortunately, 1 of each are black (I think the are the males but am honestly guessing) so the fry are not very colorful but it is clear now that there are both varieties of Koi. I have read they do not cross breed and if they occasionally do the offspring will be sterile. Is that true?

I probably now have a hundred little guys and still 4 older ones. I'm assuming natural selection took care of the others...none died (to my knowledge) on their own..except a few my kids brought inside in a large fish bowl...they obviously didn't get enough oxygen. :-( The rest have stayed in the pond and have been growing nicely; some aggressive /greedy and now 4+ inches...a couple slow swimmers, (butterflies I think) that just haven't grown to the same rate (maybe 1.5 inches). I think I have one who's fins haven't developed - I can barely see them...but it's very pretty and seems to be making it hiding most of the time in the undewater feature I have, so I've left it be.

Recently, however, a few of the fish (and I'm noticing it's spreading) have what appears to be a white film on them. It's transluscent. At first, I thought the one who is most affected (it's almost all white) was changing color, but realized after a while it wasn't that. Now more are presenting with the same thing. There is no apparent "flashing" . I've checked as much as I can online and am wondering if it's excess slime coat/mucous. I don't really don't know. PH, Nitrite, Phospate and Ammonium were fine. PH slightly elevated(7.5) but that has been normal/consistent and from what I've read that shouldn't matter.

Then all of a sudden I noticed a lot of floating particles in the water. PH was elevated (8) but ammonia and nitrites were fine. I cleaned the filter (it needed it!), backwashed the pump and made a point of removing leaves a couple times a day. PH went down to almost 7... but it spiked back up to 9 in 2 days. Nitrities were fine. Phosphates slightly elevated (which they have been pretty much for the past year and ammonia for the first time in months was slightly elevated.
I treated with some diluted vinegar (1 cup for the whole pond...1000Gallons) so I thought that would be "safe". The next day I added a pond sludge reducer. There was moderate improvement but the PH spiked again. PH this morning if anything was slightly low...first time I've seen that -- then I've not been testing as frequently before either - and I understand PH is always lower in the morning and higher at night (especially in hot climates). Just now, abnormal amounts of particles still in the water, fish congregated at the bottom of the pond - this too is not normal for them. Something is amiss.

How do I help them and prevent them from getting more stressed and/or more sick.

I've read drastic spikes in Ph can stress the fish...so I want to know if I should be doing partial water changes more regularly...the water source is ground water...it's hard but PH is 7 and no Ammonia, Nitries or significant phosphates in the water...I've not tested for chlorine (don't have test strips) or salt...but living here I expect there will be some salt and no chlorine. I am not testing Kh --haven't done due diligene on that and maybe that is what is causing the daily spikes in Ph? ;

I don't want to see my fish get sick or die. In this climate, I am aware bacteria and parasites can quickly become a problem...and have no idea how to tackle that.

What can I do to get rid of the white coating?
Also...I currently have no plants and would like to add some...not sure what to add given climate and pond depth - although that is secondary to dealing with whatever is 'blooming' and stressing my fish.

I also think I need more aeration and a better filter, but I can't do that right now...

Expert advice appreciated.

Thank you.