Every tank has probes that log oxygen, ammonia, temperature and pH, because a tank that goes wrong kills everything in it by morning. Tank 9 has been empty since the spring, refilled with clean water for testing. Its oxygen falls overnight and its ammonia rises, in the proportions a fish of about eight kilos would produce, and there is nothing in it.
I am the senior aquarist, and the water quality logs are the first thing I read every morning.
What the probes measure.
Dissolved oxygen, which fish use up. Ammonia, which fish produce. Temperature and pH, which fish are sensitive to. A stocked tank shows oxygen falling overnight when the lights are off and the plants stop, and ammonia rising a little between water changes, and the size of those movements is a fair measure of how much fish is in the water.
An empty tank with clean water and the pumps running shows a flat line on everything.
Tank 9 was emptied in April when its stock moved to the new display. It was cleaned, refilled with treated water, and left running with its probes in so the system could be tested before it was restocked.
Its oxygen falls overnight. Its ammonia rises. The numbers, run through the same arithmetic we use for stocking, say there is about eight kilos of fish in tank 9.
There is nothing in tank 9. It is a glass box of clear water that you can see straight through.
What has been checked.
The probes. Swapped with tank 10's, which is stocked. Tank 10's probes in tank 9 read the eight kilos. Tank 9's probes in tank 10 read tank 10.
The water. Drained again, the tank scrubbed, refilled from a different supply. The same numbers within two days.
The filter. A biological filter is full of bacteria and they do use oxygen and process ammonia, and that is the honest answer for a tank with a mature filter and no fish. Tank 9's filter media was replaced with new, sterile media. It reads the same on the first night, before any bacteria could have established.
The glass. Nothing on it. The tank is clear.
And the arithmetic, which I have had checked by the person who taught it to me. Oxygen falling and ammonia rising in that ratio is respiration and excretion. It is an animal.
What kind.
Eight kilos is a big fish for this collection. The tank held, until April, one animal of about that size, a grouper that had been in the collection for twenty two years and that moved to the new display and died there in May, in the way old fish do, of nothing in particular.
Tank 9 started showing the numbers on the night after it died.
The feeding.
Fish are fed on a schedule and the food is weighed out per tank, and a tank with no fish is not fed. I did not feed tank 9.
In June, one of the junior aquarists fed it. He had the round list from before April and tank 9 was still on it and he dropped the portion in, which was the grouper's portion, because the list had not been changed.
The food was gone in the morning. Not dissolved, not on the bottom, not in the filter. Gone, and the ammonia log for that night shows the rise you get after a feed, and the oxygen shows a fish working at its food.
I changed the list. I did not tell him what he had done.
What is done about it.
Tank 9 is off the restocking plan. It runs, with its probes in and its pumps on, and I test it and record it, and the numbers are in the log under the tank's name with a note that says biological load under investigation, which is a true note, and which the inspector accepted.
I do not put fish in it. Something is in it that eats eight kilos of grouper's food and breathes eight kilos of grouper's oxygen, and I am not going to give it company.
What I do.
I read the logs and I do the rounds.
The one thing I have changed is that tank 9 is fed.
The grouper's portion, at the grouper's time, dropped in by me, and gone by morning. I decided that in the summer, after a fortnight of not feeding it, when the oxygen curve on tank 9 went from the curve of a fish resting to the curve of a fish working the tank, the way they do when they are hungry, all night, and on the third morning the log showed the numbers of a fish at the surface of the water, where the food goes in, at the time the food goes in, waiting.