Lately my koi have been pointing head down and eating algae and very little of the food I give them. It started with one fish, now 3 of them. I changed the water, checked for all water pollutents, placed their pond aerated in the center of my 1,000 gallen pond, temp is about 70 degrees. I don't know what else to do. I treated with salt, melafix and pimafix, fresh water, prozipro not all at the time. Followed pkg directions. This started about last month. The fish also spend a lot of time on the bottom of the pond and behave sluggishly