Your breaking all the rules of fish keeping by not having a drop test kit , I'm suprised the shop you brought them from didnt tell you you needed one,
API do a good pond test kit cost £34.00 Uk it covers all the water perameters you'll need i;e Ammonia Nitrite Nitrate and Ph a 10% waterhange wont cut it either 40% would be better and remember to add dechlorinator/dechloromines liquid as the water goes back in.
Have you got an airpump on the pond if so add more airstones if not go out and buy one a pond airpump not an indoor pump .
Next when was the last time you did maintenance on your systems i;e filters and pond with the pond I mean any detritus from dead leaves etc if you havent then I'd thnk about setting about doing it , with a pond vacuum if you have one a net if you havent.(this is vitally important you do).
You may have got lucky in the fish telling you things are not right in the pond (l;earn to read your fish for signs of stress illness and disease).
Hopefully we can turn things around for you between us all
Dave