Problems with Water Feature

In a rectangle small flower bed, I have a small water feature which consists of a tank, sunken into the ground , the capacity of which is about 15 to 20 litres. A motor with filter is in the tank and pumps and circulates the water through a hole in a rock. On the tank is a grid and on that pebbles. 
The birds love to drink and bathe in the water. 
The problem: The water smells within a few days due to earth worms, the odd slug and wood lice falling into the watertank, and I guess this is part of the cause of the smell. I do syphon off whilst the motor is going and at the same time using a hosepipe flush through clean water, clean the filter, every few days, BUT, I still need to totally empty the tank, dry it out and start again, even after that.
It was suggested by Richard that I try using the ceramic chips, a bag will last 10 years, for 250 litres. I only have 15 litres.
Once again, the water started smelling unfresh, so dismantled again, and in the ceramic bag, earth worms are caught up in it. This proved difficult flushing them out. So they need to be picked out. Or squashed to pulp, and then flushed, or undoing the knot and dismantling to wash!
Is the net 'trap' part of the supposed function /cleaning process i.e. trapping the worms in the net? Or is this a coincidence.?
I can't fathom out a way of keeping the water feature fresh and bacteria free for the bird's safety, without such a palaver!
 
Any suggestions, please, Jo!
 
Many thanks.

It sounds as though the worms, etc are entering your water feature under the liner.  The liner will provide excellent conditions for snails and worms to thrive. 

My water feature is sealed with no gaps between the reservoir and the surrounding water capture area.  Clean water cones out if the feature and runs back in.
 

I think your liner needs to be sealed to the reservoir to ensure nothing can fall into the water.  Obviously, this maybbe tricky to carry out depending on your access to the liner and tank, but until the unit is sealed up I think you will continue to have this problem.
 

I believe the solution is prevention of contaminants entering the water rather than trying to clean dirty water. 

It may be advisable to find a reputable landscape firm to inspect the feature and give you further advice.

I hope I have been helpful.  P

lease let me know if I can be of any further assistance.

Kindest Regards

 Jo Blackwell

 Kitchen Gardener

 https://www.harrodhorticultural.com