Why Is This Tank Bucking The 'rules' Of Fish-keeping...

Yesterday we went to visit our daughter. Number two grandson has exactly the same tank as us. 
He has had it running now for about four months.
Since he and his brother gave us our tank I have been surfing the web, joining this site and generally trying to absorb as much info as I can so that when our tank is eventually cycled we know what we are doing.
I've been disappointed to find that there are very few fish that will comfortably fit into this tank, an AQUANANO 22/30 according to this site's and others community creators.

Now I was a bit sneaky yesterday. As I have been fretting about his tank I took my liquid ammonia and nitrite tests over and in the course of a conversation while the pair of us were looking at, and honestly on my part, admiring his setup and fish, he agreed to let me test his water.

He had near perfect results. NO nitrites at all, the barest trace of ammonia, nitrates lower than I presently have.

In this tank he has
One red tail shark
One corydora (the other disappeared)
Three rummynose tetras
Eight neon tetras (3 of which are young and small)

I know from all I've read that this tank is grossly over stocked and that the shark (sold to him by PaH) shouldn't even be there, let alone the amount of fish in the tank.

Now, he does NO maintenance, other than topping up the water due to evaporation and even then I've had to remind him that the conditioner goes in first or he risks his bacteria
His water is crystal clear, the water parameters I would be dead chuffed with.
He bought a plant from PaH last week that took off to the extent that within a few days he was having to trim it because it was out of the water and spreading across the tank. (~I bought three plants and they all died!!)
He has an ornament that looks like it's covered in bacteria.

When the last lot of neons went in they initially shoaled, which I told him was a sign of stress due to the shark, so shoaling wasn't something he should be viewing with any pleasure.

I downloaded some info for him when he got the rummy nose to let him know that they will probably get stressed in this little tank and not to be surprised if their colouring faded, and that they were the equivalent of a miner's lamp underground. They can be the first indicator that something is wrong.

I accept that we cannot know what is in the mind of a fish but from my observation yesterday, deliberately watching for problems, I can honestly say that the fish appear to be very happy.

They look healthy, they feed well, he has never had white spot, fungus or anything like that. The rummy nose have lovely red noses and striped fins. I didn't see a single instance of clamped fins and believe me I was looking so that I could say "I told you so" !! 
He has never lost a fish through illness, just a couple when he first set the tank up and they were so small they got caught in the filter. 
He also lost a couple due to jumping out of the tank with the help of the oversize filter that comes with the Nano and the stupid positioning of the finger hole. (No deaths I say, although it's a  puzzle what happened to the cory- no sign of it at all)

Yes, the shark does come out of his hideyhole and chases them, but they don't shoal and once he goes back into hiding they continued as if nothing had happened, no sign of nervousness or anything, they even went back to feeding. The remaining corydora even goes in an out of the hideout while the shark is in there.

Grandson has agreed that he needs to, and will, rehome the shark so I feel easier in my mind about that.

After all this long post, I suppose what I'm asking you experts is - WHY? 
How can he have almost perfect water, and I'm talking here ammonia colour that I had to really squint to find a trace of because I wanted to find it to prove my point!

He is not a stupid lad.  He is 18 years old and happy go lucky. (surviving inoperable cancer at 13 has probably got something to do with that attitude)

He has always been sensitive to others, never a bully etc and would be horrified to have it proven that he was unkind to the fish, or that he was keeping them in horrendous conditions.
When the fish look good, appear happy and not stressed in any accountable way, can his stocking really be that bad.  I know the filter (400litres an hour)probably allows for a little extra stocking but I've been trying to find something suitable for my replica tank and been disappointed.

I pretty sure the reactions on  here are going to be of horror but I've seen this tank and believe me,
for a small tank it looks good

I could say a lot more about this, maybe I will when I have time, BUT :

The filter rate is enormous, nearly 20 times per hour,  for the size of tank which I expect is a design feature so that people can keep more fish in a small tank.
 
The shark will grow much bigger and become more aggressive as it gets older, it is this that causes problems rather than a juvenile added to a small tank. But better get rid while it is still smallish.

Plants and fish go together beautifully. Fish produce two of the things that plants need, carbon dioxide and nutrients, the latter in their poo. I have found that the more fish there are in a tank, the better the plants grow. Of course, if you meticulously vacuum the gravel every week the nutrients will be lost. The rapid growth in your grandson's tank is probably because the plant is using ammonia/nitrites/nitrates, to the benefit of both the fish and the plants.

There are also very easy plants and difficult ones, in the same way as there are easy and difficult fish, the easy plants will grow almost anywhere, the difficult ones require expert care. He probably has easy ones, you may have chosen more difficult ones.

Apart from the shark, the other fish are not exactly bulky for their length, nor are they very active types. Small tetras have a lot less biomass than some fish of the same length, such as barbs, and are not as active as some similar shaped fish such as danios, both of which would produce more ammonia than the tetras relative to their length.

The corydoras that disappeared probably died and was eaten by the others, maybe this was one fish that couldn't take the conditions?

I have broken, and continue to break, many of the "rules" of fishkeeping with regard to stocking rates, water changes etc. So far I have had no problems, I sometimes think that too much fiddling with a tank does more harm than good. But I do make sure that my filters are always flowing well.

I have broken, and continue to break, many of the "rules" of fishkeeping with regard to stocking rates, water changes etc. So far I have had no problems, I sometimes think that too much fiddling with a tank does more harm than good. But I do make sure that my filters are always flowing well.

I have to agree with Richard here, I used to go crazy on my tank maintenance and was always having issues. i now do fewer, smaller water changes and only really fiddle with stuff if I have to but I do check my filters are at full flow daily. I have had no problems for a very long time now.( fingers crossed)

FC76

My original reply intimated that water changes were unnecessary; Richard W and Fishcake said they they carried out few water changes. This is not to say that it is safe for you, or your grandson, to follow suit. Please seek advice before doing so. It can be very risky for fish!

Thanks everyone.
Grandson's live and very rampant plant has only been in the tank for, well not more than a week if that.
Before that there wasn't anything apart from ornament and fish.

I think it was just that I was so convinced his tank must be totally toxic to those poor fish that I couldn't get my head round it, especially as he hasn't had any major problems and the family keep asking why we haven't got fish in the tank they gave us for Christmas.

I obviously have to be careful and tactful about saying I wanted to do it "properly" and I still think I am happier doing it fishless cycling.  His original fish must have suffered to some extent.

I'm nearly there with ours, hopefully !!