I have had a preformed hard shell pond of 50 gal in my yard for the past 3 years. It had a submersible pump with a mesh filter. it has an up to 100 gal capacity. I had 2 gold fish (installed without my knowledge from a nice neighbor when I wasn't home) and I bought a green frog. It is decorated with water lettuce and water lily. It is only 2' x 4' x 18in. deep.

It was doing great till 3 weeks ago when I found two dead baby house sparrows in it, then a week later the frog died. then 3 days later both fish were dead.

I had the water pH tested and it was very basic! I can't figure out what happened but I was wanting to put in a slightly larger one any way. so now I am faced with how to proceed.

I have it emptied and everything is cleaned.
If this was your problem what would you do??