*To add to your list
@Colin_T the Pea Puffer [or Dwarf Puffer, Malabar Puffer, Pygmy Puffer (
Carinotetraodon travancoricus)] does this as well, in regards to choosing sex. As they mature, the prospective male secretes a hormone that prevents others from becoming male. I watched it first hand in the last few weeks (dont scold me too hard) as one puffer I expected to be the male, as it was slimmer, and seemed more dominant, ended up staying female, and the largest, roundest puffer, developed eye wrinkles, a yellow belly, and turned into the dominant one.
Did you buy more fish?
sorry I am cackling maniacally at your MTS
You will buy more fish. You will buy more tanks. You will buy more fish. You will buy more tanks. You will buy more fish. You will buy more tanks. You will buy more fish. You will buy more tanks. You will buy more fish. You will buy more tanks.
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re: your link about the self sustaining fish bowl.
Suzy Shelley, a graduate from Loughborough University, created the 15-litre tank, which is packed with technology enabling fish, plants and bacteria 'to work together to create a balanced ecosystem'.
She is using a small volume of water and has a simple nitrifying bacteria cycle with the end product being nitrates that the plants use. Chances are most of the ammonia will be used by healthy growing plants and the nitrates will stay low because of that. However, the tank still has the same problem as any other tank, the reduction in minerals and an ever increasing build up of hormones and microscopic organisms that will eventually take their toll on the fish. The gunk in the gravel will eventually build up too, regardless of what the company claims.
The small volume of water will also be more susceptible to temperature fluctuation and water quality problems if something does go wrong.
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The closest anyone has come to a closed system is a reef tank full of live rock, corals and a few fish. The live rock acts as the biological filter and has aerobic bacteria on the outside that breaks down ammonia and nitrite. Inside the live rock is anaerobic bacteria that helps convert nitrate into nitrogen gas. These tanks usually have a thick layer of sand (4-5 inches) that also becomes anaerobic in the bottom and that helps remove nitrates too.
Some of these tanks have sumps that are full of marine algae like Caulerpa and these plants help remove nutrients as well. Most of the sumps have a piece of filter floss to trap debris that washes out of the main tank into the sump and this floss is cleaned or replaced regularly.
These tanks do have supplements like calcium added to them on a regular basis and are topped up with fresh water to compensate for evaporation.
People have had reef tanks set up like this for years and don't do water changes on them. However, if they test the water they will find it full of microscopic organisms that can harm the fish or corals. The only way they can remove the microscopic organisms is with a UV steriliser but a lot of people don't use them and they remove beneficial plankton as well as harmful organisms.
These tanks are usually large (1000+ litres) and have less than 10 smallish fish (average size 3-4 inches), and they have the gunk removed regularly. Comparing this to Suzy Shelley's tank, which only holds 15 litres and has a 2-3 inch fish in it and the gunk is not being removed.
Some people have small reef tanks and keep them like this and sometimes they do well but other times they don't. Generally the more water, fewer fish and lots of plants will work better than smaller volumes of water.