
Hi
Try Barley Straw, that will help to keep it clean if you don't want filters and pumps installed. Here's a link for more details
http://www.bestfish.com/barlybal.html.Try not to top them up with tap water as that makes the water react and turn cloudy too.
You may need to add more plants that produce oxygen or shade the water from direct sunlight by using shrubs but dirty water does not always mean poor quality in fact the Enviroment Agency never like to see clear lakes which indicate a possible poor quality.
There are many ways you can go but Barley is a natural way without adding chemicals which might not be good for your dog.
Regards
Stu