Welcome, but sorry we're hearing from you under these circumstances.
Please read below and answer as many questions as possible. We'll need more information before we can say anything for sure, but just from what I've read so far, it sounds like your pond is overstocked. A 300 gal. pond might be OK for a few goldfish, but certainly not for koi. If you've lost 12 GF and 4 koi, how many did you have in there to begin with and how many [and what kind] are left? And how big are they?
You said you built a "new biofilter." Is the pond new as well, or was it already established and you simply upgraded the filter? Three weeks might not be enough time for the pond to cycle, so your biofilter might not be fully functioning just yet. With a test kit (see paragraph below), you can follow the levels and you'll know when your pond is ready to add fish. And it's a common misconception that "crystal clear" water means healthy water -- it doesn't (but it looks nicer).
You need to have your water tested for ammonia, nitrites, nitrates, pH. You can take a sample to a local pet store, but it would be better if you could buy a test kit ("drop" kits are more reliable than "strip" kits) so you could learn to do this yourself.
Another contributing problem might be your water temperature. How deep is your pond?
If you can, buy a thermometer so you can keep tabs on the water in the pond -- my pond is almost 3000 gallons. Our local temps have been running in the high 90's, as well, and water temps are in the mid-to-high 80's -- that's high for GF & koi, which are cold water fish. And this temp-thing is happening to ALL of us right now -- we just have to monitor and take steps to keep the water as cool as possible.
How were the dead fish acting before they died? Did you notice anything unusual like were they congregating at the surface, gasping, or were they swimming erratically, rubbing against the sides of the pond, or jumping? And did you notice anything unusal about the dead fish such as missing scales, sores, any discolorations, "growths" on their bodies, etc.?
Here's what I would do:
- buy a test kit (or at least take a sample to have it tested) -- ASAP
- do a water change -- if your water has chlorine and/or chloramines added by your city, be sure to add chemicals the neutralize these.
- get an air pump and add some air stones
- just in case your might be over-feeding, I would either cut back or stop feeding temporarily
- find a way to add shade
- depending on how many fish you have, you might need to consider re-homing some
- be sure you have good surface water movement via you fountain and biofilter -- adding airstones will help, as well
.