You already know your alkalinity is high, 7-10 being the average range, though I have heard of some who do keep it much higher.
Not sure if you know how your alkalinity got so high so I'll share some thoughts (based on that ratio of dKh to pH)
If you have been trying to raise ph by using sodium bicarbonate (baking soda) then this can lead to the large increase in alkalinity with only a slight upward increase of pH.
You can increase pH with much effect on alkalinity by using baked baking soda, or soda ash. If you are dosing Kwalk or 2 part then you already know what needs to be done.
A rapid swing in alkalinity should not cause any problems for your fish or anemone, It can however have a disastrous effect on sps. Any rapid shift in pH is detrimental to your fish however.
Aeration as mentioned earlier will help stabilize pH, and your protein skimmer helps with this alot though simple surface agitation is generally considered the best way to facilitate gas exchange assuming there is enough "fresh" air. Tightly closed rooms will have higher co2 levels which will reduce the pH in youre tank.
Short of rapid pH, salinity and/or temp swings the most likely candidate for your fish losses is disease/parasite. Brookynella is a very common affliction for clownfish, though there are many other possibilities. All it takes is one un-quarantined diseased/parasite ridden fish to wipe out much of your tank inhabitants.
Make sure that your make-up/top-off water is up to par without chlorine or chloramine. Carbon is a good choice to remove some toxic substance from your water as well as remove tannins. If you suspect that you may have some other toxic substance in your tank, a poly-filter pad will help remove many things that carbon cant. As far as copper goes (which a polyfilter pad can remove) all of your inverts would be toasted before any fish (except several scaleless varieties) would be adversly affected.
