So this person has a few small tanks which were probably smaller than my one gallon however she seems to keep an eye on the water quality through testing. Can you really keep it good through regular testing?
Haha. Everyone here has different views on tank sizes.
With smaller tanks, it is far more difficult to keep water quality at its tip top shape. You would need to change the water almost once a day in order for the ammonia or nitrites to not spike. For this reason, many aquarists and posters on this forum are strongly against small tanks.
Small tanks provide very little options for stocking. If your tank is 20L or 5g (like Colin suggested), you would only be able to fit a small school on neon tetras or nanos. Even that is pushing it. For this reason, many people prefer to recommend making small tanks for shrimp or snails.
Overall, it is your decision. Everyone has different experiences with everything.
Personally, I think the smallest a tank should go is 5 gallons.
What are you meant to keep in tiny tanks and how do you clean them?
Fish need space to move and there are no fish small enough to live in a tiny tank with 1 or 2 litres of water. Even my baby fish were kept in 10-20 litre buckets and the fry were only 2-3mm long.
In my opinion, any aquarium that is less than 18 inches long and holds less than 20 litres of water will limit you to what can be kept in there, and it will stress any fish kept in there.
I agree with @Colin_T in that small(er) tanks require more constant maintenance than larger tanks. This is simply due to the fact that larger volumes of water are more forgiving than smaller volumes - negative things like pollution happen more slowly in larger bodies of water.
Lets face it, compared the the vastness of nature with a never ending supply of newly refreshed fresh water, our tanks represent a tiny confined space and the inhabitants live in varying degrees of polluted water. The smaller the tank, the greater the pollutant concentration.