lily pipes for eheim classic 18" high tank

They have Viv pipes on Amazon for about ~50 dollars each (inflow/outflow) and you can use Prime if you have it to get free shipping. They also have some generic ones that are 50 for a set but only come in one size so you wont be able to have the 13 and 16 mm sizes..

You can also look at cal aqua labs pipes which are supposedly very nice. You can get them at green leaf aquariums but they are only about 50 dollars cheaper than what youd pay for ADA I think.

the Viv pipe intake is only 6inches long in an 18 high tank that is a long way to the substrate. How far does the intake need to be off the substrate to have a good sweep?

I have just broken my 2nd lily pipe and now swear off glass ones altogether. Tired of moping up water that escaped before I could get the filter turned off. I suspect the expensive pipes are probably thicker and more durable than the cheap ones but they are expensive. I'm moving to DIY acrylic pipes. They are simple to make if you've got a heat gun and much much cheaper than buying the glass ones.

Just use 13mm for both in/outflows. Plenty of knockoff brands w/ 12"+ inflows in that tubing size. You can get a 5/8" to 1/2" plastic hose reducer at Home Depot for cheap. I've run my 2217 like that for years w/o any problems, and an unnoticeable reduction in overall flow.

Viv pipes are nice quality, I have a couple, and to be honest most glass pipes are pretty close in quality, so I don't see the justification in paying the ADA prices. But glass pipes are very fragile, so handle with care. But they're nice to look at, when they're clean.