With things like tubing, media, gravel, crushed coral and other physical stuff in tanks I tend to get them from sources that provide them specifically for aquarium use, unless a member here or some other info leads me to feel sure the material will be safe for aquarium use. Its true you can get ripped off, with excessively high prices for something that's the same thing from a big box or hardware store, but without specific help its probably better to be safe than sorry (although even aquarium sources can be unsafe in rare cases unfortunately.) For instance, an example that springs to mind is the recommendation to go to walmart or a crafts store to buy big bags of "pillow stuffing" polyfloss that is plain and doesn't have fire retardants or other extra chemicals -- this was a recommendation made here to save money.
It could be that AquaClear themselves might have other size siphon tubes, or any of the HOB makers. It would be good to carefully observe whether the end in the filter needs to fit tightly or not. Tolak is so experienced, he might know straight out. I agree with him that its a really convenient feature to have a long intake tube so that the filter can just keep running during even large water changes. Mine can keep running up to about 80% change.
Its also true though that keeping it running is not at all essential. Most beginners are much more worried about bacteria die-off than they need to be. When the flow of food (ammonia) and fresh water stops going through a filter, you still have at least 5 hours and probably more before you'd detect any die-off I'd say, at least the 5-hours being ok is something I've directly experienced. The next stage of worse is if the media is not submerged in tank water, but again, if kept wet it will still do fine for a very long time and indeed that is how people ship mature media to each other. The worst thing is if it dries out, then you get pretty rapid and severe die-off of the colony (although even in this case you can be fooled and the centers of the colony can be still moist and the few cells that haven't died can be a big boost to re-cycling the filter, it just varies with the severity of the dry-out.)
Yes, media is an important topic. In your own case I'd bet pieces of sponge in rough and fine pore sizes would make a great biomedia for your AC/topfin and would give you the flexibility to move pieces for your other needs. Tolak could verify/correct me. Other biomedia to learn about are ceramics, both pebble and ring shaped. The desirable feature of these is that they provide good bacteria clinging surfaces and yet are like stone and will last for years, letting you simply dunk them in tank water to rinse away the excess debris. The ring idea is to randomize the little directions the water takes as it passes through the filter, with the result that the water can't find a "path of least resistance" that would allow it to get by with less filtration. In smaller HOBs, there may not be enough room to build the minimum layer size for an effective ceramic ring bed (not sure thus, about the AC20.)
BTW, for "getting the basics down firmly" with the filter media topic, you should organize your thoughts into three categories: mechanical filtration, chemical filtration and most importantly, biofiltration. The stuff we've discussed functions as both mechanical (debris catching) and biofiltration.
~~waterdrop~~
ps. Question for Tolak et.al. if he happens back: I hadn't ever thought about the filter flow rate slowing down as the tank level recedes, would that be just the siphon effect (ie. less push from the water above the filter intake point?) Interesting.
