For my grow beds that are fed from a 1 1/4" indexing valve with a Quite One 4000 pump, I am using 1 1/2" drain plumbing from the individual beds and that connects up with a 3" common drain line for multiple beds.
And I must admit that all my media bed drain plumbing has been 1 1/2" except where in the past I did use some 1" loop siphons but I had to have a ball valves on the inlet of the grow bed to get the flow balanced right.
For a SLO drain out of a fish tank I would recommend larger plumbing. I personally like the 3" pipes for slow drains on large fish tanks but I can see where that could become a bit costly. I have done a 2" slow drain from a 100 gallon fish tank and that worked out ok for me. I know people who have done double 2" drains as well. Here is a trick for use with 3" plumbing though. The uniseals are generally for the schedule 40 type plumbing though the thin wall pressure pipe will also fit it if it fits the same size PVC fittings. However I found that the 3" sewer pipe and fittings are less costly than the 3" schedule 40. So I found that a small bit of 3" pipe through the uniseal and then adapted to fit the 3" pipe was a good compromise. I even found that sometimes one can get a 3" sewer pipe coupling to fit through the uniseal and avoid the extra adapters but that is trickier.
I've not had problems with any uniseal leaking unless the hole was mangled a bit or extra tension is pushing the uniseal causing a leak.
Just make sure where you drill the hole will be fairly flat (don't try to install a uniseal in a corner or ripple) Make sure there will be space for the flange of the uniseal and that you will have the leverage to push the pipe through. There are places in the bottoms of the tanks where the ribbing probably wouldn't get in the way but it was just far easier for me to go out the fronts of the tanks just to keep the plumbing easy to access and work on. When double checking the hole saw with the uniseal, you want to make sure the uniseal will not actually fit inside the hole saw because the thickness of the blade would make the hole too big. The hole wants to be just a shade smaller than the uniseal so that you will actually have to flex the rubber to get the uniseal into the hole. Anywhere selling the uniseals should give the proper hole saw size for making the hole.
To locate a dealer for indexing valves
Thanks for the advice on drain plumbing sizes.
Is there something special about a "slow" FT drain? I was going to have mine overflow near the top of the tank, but the water will be drawn up from the bottom of the tank (to better clear solids).
The dealer map wasn't working in Safari, but it works now in Firefox. Although you're not my nearest dealer, I would prefer to give you my business in appreciation of all the advice you've provided. Are you able to ship one to me in AZ? I will already be buying a pump and probably Uniseals from you.
Also, can you comment on my gravity-modified question?
SLO= solids lifting overflow. Don't make it draw from too near the top of the tank. I've seen a few situations where the overflow hole though the side of the tank was placed right below the rim but due to some restrictions in the water path, the tank would fill up faster than the overflow could take the water away. You can always adjust the height higher using plumbing but if the hole is too high up the side of the tank, it would require cutting a new hole to make it lower.
Other questions being answered in private message