My understanding of the nitrogen cycle seems off? Please illuminate!

[Mayank]

Ok, so I've been struggling with this for a few days. I thought I understood the cycle (I am a Chemical Engineer originally).

Fish eat food, fish poop/pee/respire or however, end up making ammonia.
One type of bacteria (nitrosomonas) converts ammonia into nitrite. Another type (nitrobacter) converts nitrites into nitrates.
Nitrates is food for plants and either needs to be absorbed by plants in the pond or becomes free ample food for algae (which is always there more or less even if you have clear water and the pond has cycled) and, voila, you have a bloom.
SO, the questions:

1) Although the algae helps remove the nitrate, it also removes much oxygen in the process, so ideally you want to stock/feed fish only as much as your plants will allow based on their nitrate requirement. So, how do people maintain crystal clear water in "sterile" looking ponds with koi? I saw one such inside pond at "The Pond Place" and it had 2 ft koi and crystal clear water. I asked how? They said it was UV light. OK, I get that, it kills the free algae. What about the poo/pee stuff though. This pond had zero plants (it was indoors). I am assuming it had good bacteria, so no issues with ammonia and nitrite, but then wouldn't there be nitrate and it would build up even if they had a bottom drain and they were doing water changes? Is there any other way to neutralize the nitrate?

2) Let's say you use the UV light to get rid of free algae (even though some contend that algae is actually fulfilling a purpose which I am assuming is by removing the nitrate which though not as bad as ammonia or nitrite can still hurt the fish if levels go high?). People have said that crystal clear water does not mean it's good for the fish if achieved artificially (i.e. without pond cycling naturally). Lets say you did it anyway (clear water through UV). If this was a bad situation, would this NOT show up on the water testing??? And if the water testing results are clear then is it still an issue?

3) After we used MinnFinn to treat the ich on our fish, we had an algae bloom. The product does say to bypass filter if possible to prevent killing the good bacteria but for our setup it is not possible. Anyway, we were told that the algae bloom was because the medicine wiped out our beneficial bacteria and we needed to add more which we did. Here is the question though: if the good bacteria were wiped out the how did the ammonia become NO2 and how did the NO2 become NO3?? Our testing showed zero NH3, zero NO2 and zero NO3. So how can the algae bloom if there was no NO3, only nitrate and NH3???

Sorry for the rambling, but I really want to understand the mechanics of this cycle and how it works.

 

[IPA]

You have a good understanding of the cycle. The giant Koi aquariums, aka koi pond, sterile of plants will compensate with all types of filtration, basically think of a waste water management facility for koi. They use biological, mechanical and UV sterilization. Flushing the system, filters, is important and then topped off with fresh water so basically a small water change is preformed often. There are even algae scrubbers to remove nitrates. In the garden pond, bog pond, the idea to to remove the nitrates through plants. Many people use a combination of plants and other filters. A proper bog only may need a skimmer, if that, or mechanical means to protect the pump. I have a cage style intake filter. Any leaves or other material drawn in eventually rots before being pulled into the system.
Oxygen exchange happens only at the surface (even bubblers and it really isn’t the bubbles adding oxygen to the water, in contact so short a time, but the movement and disruption at the surface). Waterfalls, moving water across the surface, intakes below the surface all add oxygen so it constantly replenished. Sure, if the surface of your pond is covered in hair algae it will deplete oxygen but mostly because it is preventing the exchange of gas at the water surface to air mechanic.
As for the bacteria, while many medicines, freezing temperatures, reduced oxygen will kill of the bacteria, it probably isn’t going to kill all of it. It isn’t just growing in the pea gravel or biological media but on every surface. Just like most bacteria, it bounces back quickly. It is right to monitor ammonia and nitrite leaves, pH and hardness to make sure it has recovered. pH shock is common after treatments so keep monitoring and get that KH up; simple solution 1 cup baking soda to 1,000 gallons of pond water.

 

[Mayank]

You have a good understanding of the cycle. The giant Koi aquariums, aka koi pond, sterile of plants will compensate with all types of filtration, basically think of a waste water management facility for koi. They use biological, mechanical and UV sterilization. Flushing the system, filters, is important and then topped off with fresh water so basically a small water change is preformed often. There are even algae scrubbers to remove nitrates. In the garden pond, bog pond, the idea to to remove the nitrates through plants. Many people use a combination of plants and other filters. A proper bog only may need a skimmer, if that, or mechanical means to protect the pump. I have a cage style intake filter. Any leaves or other material drawn in eventually rots before being pulled into the system.
Oxygen exchange happens only at the surface (even bubblers and it really isn’t the bubbles adding oxygen to the water, in contact so short a time, but the movement and disruption at the surface). Waterfalls, moving water across the surface, intakes below the surface all add oxygen so it constantly replenished. Sure, if the surface of your pond is covered in hair algae it will deplete oxygen but mostly because it is preventing the exchange of gas at the water surface to air mechanic.
As for the bacteria, while many medicines, freezing temperatures, reduced oxygen will kill of the bacteria, it probably isn’t going to kill all of it. It isn’t just growing in the pea gravel or biological media but on every surface. Just like most bacteria, it bounces back quickly. It is right to monitor ammonia and nitrite leaves, pH and hardness to make sure it has recovered. pH shock is common after treatments so keep monitoring and get that KH up; simple solution 1 cup baking soda to 1,000 gallons of pond water.

Thank you for such a detailed response. That was very helpful. Our test kit only tests pH, high range pH, NH3, NO2 and NO3. Our test strip does pH, alkalinity, total hardness, NO2 and NO3. We use both modalities. Do not have a KH test. Is that a problem? Our pH is usually 8.2 (at least every time we've measured it) and the alkalinity and total hardness is always on the high side. Amm, NO2, NO3 always zero.

 

[Lisak1]

@IPA nailed it. The only way to keep fish healthy in a pond without plants is with supplemental filtration to increase the biological filtration that the plants and algae would normally provide. That's where those bottom drains and trickle filters and shower filters and whatever other filters you can name come in - which require frequent and regular cleaning and maintenance. People who have koi ponds for the love of the fish consider all of that part of the hobby. They test water and make adjustments to maintain the chemical balance - much like people who love tinkering with cars or computers.

I enjoy the fish in my pond but I also love the plants - for me, it's an extension of my garden. And since I'm somewhat of a lazy gardener - I'll give any plant a chance, but I don't baby anyone - I'm also a rather lazy ponder. I want my pond to pretty much take care of itself. I have a bog filter - more garden! - I don't test my water, I feed my fish when I remember, I fool with my plants when I feel like it... water garden pond vs dedicated koi pond. The problems start when people criss cross the two.

 

[addy1]

I enjoy the fish in my pond but I also love the plants - for me, it's an extension of my garden. And since I'm somewhat of a lazy gardener - I'll give any plant a chance, but I don't baby anyone - I'm also a rather lazy ponder. I want my pond to pretty much take care of itself. I have a bog filter - more garden! - I don't test my water, I feed my fish when I remember, I fool with my plants when I feel like it... water garden pond vs dedicated koi pond. The problems start when people criss cross the two.

Ditto to all you said, except I hardly every feed my fish and only goldfish/shubunkins, refuse to have koi.