Makes at least three of us who were hoping to see a thread about someone building their own backyard AFV. 
I was going expecting a Killdozer thread. 
[quote=“TriPolar, post:14, topic:850276”]
Goldfish and guppies can survive quite well with some occasional water cleaning. Small sucker fish, snails, and some others will be fine. [./QUOTE]
And usually the cory 'catfish".
A goldfish or 2 or a betta will do fine in a large fishbowl, more fish need real plants.
But yeah, a bubbler will help.
The thing about filters is that you can keep more and a larger variety of fish.
Well, at least four of us were looking for a discussion an cheap-ish DIY armored vehicles.
As for fish tanks, a childhood friend had a 30 gallon plastic garbage can buried in the earth under the 2nd story deck, with the top 2" above soil level. Why? I have no idea. His father installed it. My friend filled it with rain water & put 2 each gold fish in it. We were about 13 at the time.
Fast forward to grade 12, our senior year in HS. I house sat the family home while the entire bunch went on a three week vacation. I was sitting in the shade reading a book beside that garbage can when a large fish came to the surface & snagged a dragonfly off of the surface of the water. The fish was about 24" in length. I had no idea gold fish could live that long or that they could get that long.
When they returned I mentioned the fish. They said that it was one of the original fish & that they had not fed it in years. The water was green & some frogs lived near, or in, the garbage can.
If one wanted a budget fish tank, it does not get much lower cost than that. It is not pretty to look at, but it is cheap.
Goldfish do NOT as a rule do well in an unfiltered bowl. They are incredibly dirty and produce a lot of ammonia which is a killer. They also can get very big very quickly if you feed a lot. The recommendation for goldfish is at minimum 10 gallons per fish.
In any tank the most important thing is getting the beneficial bacterial cycle going and keeping it stable. A decent filter, substrate, and plants are keys to that. The good bacteria grow in the filter medium and the water cycling through the filter feeds the bacteria and keeps the water chemistry at a healthy level. Water changes are still necessary, but how often depends on how densely stocked the tank is and how healthy the cycle is. There are test kits that help keep an eye on that.
If you were interested in goldfish this is a pretty good resource: https://puregoldfish.com/
^Saje is correct. Overpopulating the tank will lead to disaster. When I was running tanks, I always adhered to the " 1" of fish per gallon of water " rule. So a well filtered 20-gallon tank can support 20" of fish just fine.
I believe this rule holds for fish that run less than two inches long.
My experience is that the surface of the water will grow crud (I believe it is protein) if it doesn’t move a bit. An airstone and small pump will provide for some water movement.
Regarding sucker mouth fish, plecostomas (sp) grow too large for most tanks. True Chinese algae eaters become large and aggressive. Otos are small, active little catfish.
One gallon per inch of fish is very different from 10 gallons per fish when we’re talking about goldfish, so I don’t see how you agree. Again, I had one fish in a five-gallon tank. He was maybe an inch long when I got him and maybe three inches long when he died, so I followed the latter rule but came well short of the former. But with that filter he managed to hold on longer than some of my friends’ dogs, so…
You could always start your own thread.
The closest I found was What kind of license do I need to drive a tank? from 2001, which almost wandered into building a tank. That, and a photo of a Smart Car, er, conversion.
Sorry, Kovitlac, I know very little about aquarium tanks, though I did have a gerbil in one as a kid.
Yes, goldfish are very dirty, and grow very large. They are better in outdoor pools than aquariums.