photobucket,you can buy swing check valves that mount on the vertical. They are fairly inexpensive.
Or a spring activated check valve can be installed either horizontally or vertically.
"Horton"

Okay then, a spring activated check valve. Good to know what to look for a the hardware store. The local help is not very helpful a my outlets.
Thanks.

Pondbucket, You can use a spring check valve but they do require more force to open and can add to the head pressure of the system. There are lots of swing check valves that will work in the horizontal position. The will say on the check valve if installed horizontally this side up. Installed horizontally they do not work quite as well as vertical but they do work.
Mike

I'd use a ball valve and manually turn it when needed. Check valves work great in clean water, like city water clean. Even a bit of bacteria slime could cause a leak and it wouldn't take long to empty the filter.

Same price as a check valve and you get full flow which you don't get with most check valves.

I have a 2" PVC check valve mounted horizontally (feed 150 gal skippy filter). Has a rubber flapper and arrows for proper orientation. The flapper/seat is at a slight angle - not perpendicular to pipe. Arrows provides flow direction and up orientaiton so flapper at proper angle. will work vertical also. when installed properly works fine (for 4 years).

Water is not city water clean but has healthy algae growth, enough to provide snacks for my "tenants". Piping also not pristine, but backflow never an issue. I think i have enough back pressure to seal up against bit of internal gunk & slime.

I'm glad to have all this input. I've got my pipes uncovered and I will probably attempt this soon.

Waterbug_G, I understand your thoughts with the manual valve but it won't work for situations when the electricity might get shut off (loss of power), and that's probably my second most common concern after maintenance on the filter.

Still has me thinking...

Waterbug_guy makes a good point about the possible malfunctioning of check valves, where sludge, etc, can sometimes cause them not to seat and seal properly.
I find that taking them apart and cleaning them in the fall/spring, helps eliminate the problem of a build up of algae/sludge etc around the flapper or ball.

It comes under general maintenance, when I clean the pumps impeller etc, I also do the check valve and flush out the piping with a bottle brush attached to a pull through wire.
"Horton"

If automatic shutoff is desired then check valve is one way to go. Lots to choose from. I've added a recommendation for Plumbing Supply below.

I'd still add a ball valve in addition to the check valve. They even make a combo unit which seems odd until you've had a few check valves not work completely. Yes, certainly, there are many, many cases of them working. However that doesn't mean some don't fail. I assume anyone going to this kind of trouble wants the best possible results.

The other choice, and the one I prefer in such cases, is to reroute the pipes.

One route is to raise the out flow pipe inside the filter up to just below the water level. The filter won't drain below that level. But there is rarely room in a filter for this and you can't drain the filter unless there's another drain which is handy for cleaning.

Another route is up vertically and then right back down on the outside of the filter. Most filters can handle the extra flow restriction. The top of this upside down trap has to have a way to stop siphon which can be a tee and an open riser, like a vent in a home. Can be unsightly of course. Normally the riser is very short, a couple of inches. This home made "check valve" is cheaper and will never fail.

However, plumbing fun aside, I have yet to see a water garden that needs a bio filter since the entire pond is a bio filter. So I would start with finding out if the bio filter is needed at all. But I certainly don't know this pond and most people like cool stuff attached to their pond whether or not needed.

Here is a link that might be useful: Plumbing Supply