Dead Fish With Bent Spines Or Kinked Backs.

Bent spines/kinked backs are typically a symptom of an insecticide contaminating your pond.Losing the odd fish to a deformity can be common,but to lose more than one in the same way suggests a classic ‘organophosphate; type issue.Orfe are particularly prone.However,the lethargy might point to them dying simply of old age
A change in behavior(rather than body shape)is a key indicator that there is an issue in your pond – usually caused by water quality – and typically affects all fish in the same way.
Continue to manage your pond as you have.Never clean it out too much,as a stable pond environment is the best you can offer your fish for their long term health.