kscans

Chapter 270 - Trial and Error I

"Only five days and it has already come to this... This might be tougher than I expected."

I grimaced at the scene before me as I checked the fish tank first thing in the morning.

Incidentally, the cubical fish tank was a product of the magic die Noel had created. The magic metal, which did not deform under high heat, was perfect for glasswork. Thanks to that, I was able to make a frameless tank with a highly immersive feel.

That said, the glass was thick compared to tanks from my past life, and I had made it a cube so the water pressure would be evenly distributed. And, of course, it was heavy. Its size was five liters at most.

The small fish that had greeted the morning of their fifth day in this small world were clearly losing their energy. Some were swimming upside down, their bellies facing up, while others frantically darted up and down the tank. Some lay on the gravel at the bottom, moving only their gills weakly, and there were others that could not even do that.

"They've lost their spirit. What's the cause?"

Just as I thought I heard a sudden voice, Maytyl peeked over my shoulder. Today she was wearing a loose, off the shoulder dress. I thought it suited her, but for her to approach me so closely, prioritizing her curiosity... Her defenselessness was rather stimulating for me.

"The first thing to suspect is the water quality, but..."

I focused my attention on the underwater world before me. For a fish, water is its entire world. The basic factors of water quality to consider were the oxygen concentration and the pH level. Then there were the various minerals, in other words, the water hardness.

"I'm using water straight from the pond, and if the dissolved oxygen was low, they would be gathering at the top."

I looked at the individuals that had managed to retain some of their energy. Since oxygen dissolves into water from the air, the oxygen concentration is highest near the surface. If there was a lack of oxygen, the fish should have been gathering near the water's surface.

"Next is the pH."

I dropped some water from the tank onto a thin, rainbow colored strip of paper I had brought from the Central Garden. It was a dye sensitive to various water qualities that I had asked Vinaldira to prepare. As far as I could tell, the color had not changed from the first day. It was less reliable than the water quality test kits from my past life, but it seemed there were no major changes in the pH or hardness.

Of course, water evaporates, so minerals like magnesium and calcium become more concentrated. But that was unlikely to happen in just five days.

The fish tank in my university seminar in my past life had been maintained for nearly half a year just by topping up the water.

"They're surely not getting enough food."

Maytyl said this as she turned her nose toward the smell of baking bread wafting from the kitchen.

"...The food is likely the cause, but it's probably the opposite of what you think."

Maytyl looked at me, unable to understand my words.

"I think the water has been fouled by their waste, which is the result of that food. First, the population density in the tank is much higher than in the pond."

There were ten fish in just about five liters of water. That's two fish per liter. Considering each one was half the size of a medaka rice fish, that was not a lot for a home aquarium.

But compared to a natural environment, it was extremely crowded. Even if the only living things inside were fish, the density was likely more than ten times that of the pond. On top of that, in a natural environment the water circulates, but in a tank, it is stagnant.

"I understand that. Livestock get sick if you crowd them too much."

Maytyl looked at the bottom of the tank. Of course, there was a fair number of small, slender things lying there. I wondered about the appropriateness of such a topic in a conversation with a female student, but Maytyl did not seem to mind at all. In that respect, she was truly a great help as a research partner.

"No, the most important part is what you can't see. Um, in human terms, it's the other one."

"Ah, u..."

"I'll continue my explanation. So, the end result of the food that the fish excrete is a poison. That's why they expel it, but..."

I was the one being considerate.

As living creatures, they eat, digest, and excrete. Among the substances discharged from a fish's body, ammonia, a decomposition product of amino acids, is highly toxic. Humans convert it into relatively harmless urea in their bodies and concentrate it. Birds, including horse-dragons and dragons, excrete it as uric acid. That's the white part of bird droppings.

Ammonia dissolves in water, and terrestrial creatures evolved their methods because they do not want to lose precious bodily fluids when they excrete ammonia.

However, fish live in water. With a few exceptions like sharks, most fish release ammonia directly into the water. This is not a problem in a natural environment where the population density is low and the water circulates. But inside a fish tank, the ammonia concentration rises to a level that fish cannot survive in.

"Based on what you've said, does that mean we just have to change the tank water and clean the droppings from the bottom?"

"Well, that's one way to do it. But it takes time and effort for a person to care for them, and the cleaning itself is a burden on the fish as a change in their environment. Most importantly, it's counterproductive to my goal."

If we changed half the water every day, the ammonia concentration would theoretically be halved. Another method would be to submerge charcoal in the water. The charcoal would adsorb the harmful substances. In the terms of my past life, it was "physical" filtration. But that takes effort and money. Only nobles or the very wealthy who could afford to station a servant by the tank could enjoy an aquarium.

"If you don't change the water, that poison you called ammonia builds up. If you do change it, both the owner and the fish dislike it..."

Maytyl tilted her head in confusion.

"Actually, you can make the tank itself have a mechanism to purify that poison. In fact, that's what is happening right now."

"There it is. Your unseen knowledge. Let me hear it, this mechanism you speak of."

Maytyl's eyes gleamed.

"Yes. Things produced by living organisms should be processed by living organisms."

It was so called "biological" filtration. Filtration means to strain a specific substance from a liquid, but the nuance is a little different in this case. The goal of reducing the unwanted ammonia from the water is the same, but instead of removing it, you change it into a different form.

The principle itself was in science textbooks from my past life. It uses the nitrogen cycle. For example, when animals die on land or in water, their proteins decay and produce ammonia. But neither the soil nor the water becomes toxic. That is because they are equipped with a mechanism that uses the power of bacteria to convert ammonia to nitrite, and nitrite to nitrate.

In fact, even in this tank, the ammonia excreted by the fish is gradually starting to be processed. It is just that at this stage, the rate of ammonia increase is faster than the rate of processing, so it cannot keep up. And so we move to the next stage.

"How is the tank I left in your care?"

"The empty one? It's running smoothly. Just fruitlessly churning water with nothing but gravel and food in it."

The same thing can be done in the fish tank. The device for it is already prepared. It is the screw I asked Maytyl to make, and the gravel laid on the bottom.

The tank I entrusted to Maytyl has a thick layer of gravel, and a pipe has been stuck into it. The pipe is made of magic metal, inscribed with the formula for the battering ram, and has a screw structure fitted inside. With each rotation, the tube sucks up water from below the gravel layer and releases it at the top of the tank. In other words, the tank water circulates through the gravel.

"This gravel and screw are important. With these two, we can increase the tank's capacity as a purification system dozens of times over."

Bacteria, in this case nitrifying and denitrifying bacteria, adhere to the surfaces of objects and multiply. In a tank with only water, the only surfaces are the bottom and the walls. But if you lay gravel on the bottom and circulate water through it, the surface area increases dozens of times over, by the surface area of the gravel. If you circulate water in a state where food has been left to rot and produce ammonia, a large number of bacteria will multiply on the surface of the gravel.

I picked up a single piece of gravel from the water. Perhaps because I used pond water, the surface was already slimy. This slime was the biomorph, the layer of bacteria.

"Oh, let me feel it too."

Maytyl took the gravel from my fingers. She moved her fingers and said, "It really is slimy."

"Hey, be careful."

I grabbed Maytyl's wrist without thinking.

"Wh, what was that for all of a sudden!?"

"Well, your nice clothes will get dirty, you know."

I let go of her hand and pointed to a drop of water about to fall from her palm.

"Ah, oh, right. Thank you."

Maytyl slowly returned the gravel to the tank. Then, she wiped the water that had dripped onto her wrist with a cloth I handed her. Her cheeks were slightly red.

"To continue the story. First, the Mucus that feed on the ammonia released by the fish increase, and they change the ammonia into something called nitrite. Next, the Mucus that feed on that nitrite change the nitrite into nitrate. Nitrate is much less toxic than ammonia. That's the kind of system that develops on its own."

"...So, it's about those minuscule Mucus you mentioned. It's the mechanism by which animal droppings and corpses return to the earth. Wait, could this also be related to fertilizer?"

Maytyl showed her understanding. "This is what happens when I let my guard down," she muttered.

"If anything, I should be the one saying that."

Even though the use of livestock manure is practiced in this world, for Maytyl to figure that much out from my explanation was, in a way, frightening. No, I should say it was reassuring. After all, the other purpose of this product was...

◇◇

"It's been a week, and they're still so full of life."

"Looks like the biological filtration has started up smoothly."

The fish were swimming energetically in the tank. I had added more from the pond, increasing the population density to one and a half times what it was before. Using pond water and gravel from a stream must have helped it start up quickly.

"And with the gravel, it feels like we're looking inside a river. It's a little strange to see it from the side. Anyway, does this mean we don't have to change the water anymore?"

"No, the fish waste has only been changed into a less harmful form, it hasn't disappeared from the water. In the long term, problems will arise. Minimal water changes are necessary. However, the frequency of water changes can be significantly reduced. We can probably maintain it by replacing about a fifth of the volume once a week."

Nitrate is far less toxic than ammonia, but it is just as harmful if it accumulates too much in the water. Eventually, the accumulated nitrate will corner the fish. There is actually a way to completely detoxify it by converting the nitrate back into nitrogen molecules, in other words, gas, but that requires anaerobic conditions, so it is difficult.

"I'll take one more step. It's related to what you said earlier, Maytyl."

"Something I said?"

"Yes. I'm going to add something to the tank. But before that, I'm sorry but..."

I looked at the metal tube that was serving as a simple pump. A smaller fish approached it, then hastily changed direction, flustered by the force of the water.

"So I just need to adjust the output to be strong enough for water to pass through the gravel, but gentle enough for the fish. Is that right?"

Maytyl took out a spare magic metal tube.

"That would be a great help."

"Leave it to me. I'll adjust it as quickly as possible. I'm also curious to see what you'll do next, Ricardo."

Maytyl said, rolling up the sleeves of her lovely outfit.