actually, using two 45 degree fittings with a few inches of separation in between does significantly reduce back pressure versus a 90 degree elbow. Two 45 degree fittings with little or no straight pipe between them, however, isn't much different than a 90 degree elbow in terms of back pressure. And 'sanitary bend' elbows are not available in anything smaller than 1 1/2" I think, because nobody ever uses anything smaller for residential drain pipe.
The long sweep 90 degree elbows that you are referring to ARE in fact available in all sizes of PVC pipe down to 1/2" 'trade size' in both Schedule 40 and Schedule 80 wall thickness. However, they're hard to find at plumbing supply houses since they are actually classified as a 'PVC electrical conduit sweep elbow', which you're much more likely to find at a local electrical supply house. One online source is
http://doityourself.com/store/6552616.htm (you can search around for the other pipe sizes).
Long sweep 90 degree PVC conduit elbows create far less back pressure than either a standard 90 degree elbow or two 45 degree fittings with or without straight pipe in between. They're not very expensive either, and if you are planning on having several elbows in your system plumbing, using the long sweep PVC conduit elbows will result in a significant improvement in your overall flow rate.
As for PVC glue I like to use CLEAR primer plus CLEAR glue, which are available at all big home supply places if you look carefully at the labels. Any glue products which are safe for drinking water piping are safe for aquarium use. However, some primers and glues have pigment added which can leave a technicolor mess after gluing, which is no big deal in a basement but potentially looks awful around an aquarium.
It's also supposedly possible to 'dye' the outside of PVC pipe any really dark color, by mixing ink with clear PVC 'cleaner' solvent or clear PVC primer and pouring/soaking/wiping down the whole pipe assembly with the ink mixture after initial pipe joint gluing and drying has been completed. The pipe assembly needs to be set aside to dry again after dyeing though, then rinse super thoroughly. Disclaimer : I haven't personally tried this. However, dyeing sounds like a lot better alternative than trying to paint the outside of PVC pipe with epoxy paint because of the risk of paint flaking/chipping.
If you're looking to wind up with a real 'showpiece' result, and if you don't mind spending a few extra dollars to achieve it, dark black fiberglass reinforced FDA grade polypropylene pipe, pipe and hose fittings, bulkhead fittings, and valves are also available in all pipe sizes. One online source is
http://www.beevalve.com/Bee_Valve_NavCat.pdf (go to page 34 for starters)
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