Hi KoiGuy. Yes, some of the filters did have flush valves but the algae was so sticky and slimmy that the flushing didn't do enough to remove the sludge. On the filters that had no flush valves, I had to dump them and I found that 1/3 of the lava rock at the bottom of the filter was just covered in this sticky, slimmy sludge and I can't tell you how long it took me to clean the lava rock of this algae.
So, for me, the upflow bio-filter was a disaster because of all the sludge. Now, with the downflow we are using, I never have to clean the bio-media (plastic scrubbies), the only thing that I have to clean is the window screening at the top of the filter where debris from the bottom of the pond is trapped and is very easily removed. On some of the filters with bottom drains, I just open that drain and flush everything out. The only thing that comes out of the filter is a find brown particulate which I suspect is very normal. I also used to by a product called Crystal Clear to start off my ponds in the spring and found that this product in it's freeze dried form was the same colour as the light brown particulate that is in my filters. I wonder if there is any connection.
Your's Koily, Lorraine