A school of the tiny Boraras will enjoy much swimming space; less than 40 or sixty liters should not be considered. This helps with water management which is essential for successful aquaristics. Nevertheless, these fish are used to streaming waters. You must imitate that by a) streaming filtration and b) water changing as often as possible (once a week at least 10 to 20 percent is very advisable).
Now Paros: Even two Paros only, an adult pair, will represent more living mass than 20 or thirty Boraras. And in nature they live in (slowly) streaming waters, too. Nevertheless, their behaviour scheme is entirely different. Even in nature, in a streaming water body of huge space and big quantities, they are completely used to a tiny living space. When the male has found a fitting cave built of a leaf or a piece of old wood, it is happy and leaves this cave only for searching a female or near food or to put a rival to flight. Females are less restricted this way, but nevertheless they search for males with caves, too. And when bpth meet, their life is fully centered around that tiny habitat, their living room. When they spawn, this is entirely restricted to the tiny inner space of the cave; only the male is occasionally looking around for rivals or enimies. After spawning, the female leaves the cave but lingers near to it in order to take the former duties of the male and to defend the cave with male and the clutch and the larvae. When the latter get mobile, they try to leave the cave and the male cannot stop them, although he tries at first. Finally, he surrenders and leaves the cave or – more often- tries to attract another female for the next spawning or – mostly – the same female (if it has not been eaten by an enemy in the meantime).
This means: Paros have very little need of space. The only have need of very clean blackwater with stable values. As aquarium practice proves, they even do not really need the constant weak flow of the water. But they need water of best quality. This must be considered by choosing the size and the technical installation and care of a tank.
A Paro-pair is quite happy with a tiny space around that necessary center, the cave. This was the receipt of Allan Brown, one of the best specialists of these fish, observed in nature and imitated in the breeding tank. He used 5-liter tanks and had the best results for many years. He produced huge amounts of all available species, including the true deissneri or allani or rare variants as spec. Kota Tinggi. I use 10-liter-tanks (factually 12 liters) because this is a standard size for cheap industrially produced tanks; see my often published array of 24 tanks, containing altogether less water than is contained in many single living room aquaria of today. The tanks of Walter Foersch were not much bigger. The tanks of Günter Kopic, German master breeder who bred most species for the first time in the eighties and nineties, contain 40 liters.
When I recommend 20 or 25 liter tanks I recommend them because I know the laziness of humans, me included. If you will have best results and observe what is happening, no tank must be bigger. Bigger tanks are dangerous. You miss many important things, you have difficulties in feeding the tiny young adequately (if you will not overfeed, because of water quality) and you can hardly cope well with difficulties as illness or diseases or noxious developments that you have to correct for saving your valuable fish.
But the really important thing is water quality. To imitate the constant flow of best and clean blackwaters the aquarist has only one choice: that’s water change, frequent water change. You cannot reach this aim by filtration. Water change is essential.
Allan Brown, as his wife Barbare characterized him, was “a water changer”. And every successfull Paro-keeper and breeder must be a water-changer. The biggest problem with a much bigger tank for Paros is not that you miss many of their behaviour but that you deceive yourself by thinking: o, then I don’t need to change the water too often. Bigger tanks induce laziness. And that’s wrong. If you decide for Paros, you better decide for a small tank not more than 25 liters (or fifty at most, but for most intentions this is too big already; and I don’t like those people who want to create a “natural environment” without knowledge what this means for Paros. Paros are endangered species; Simply “keeping” them in a nice aquarium is not adequate for them. Then you can decide for other fish that are not endangered. You should enjoy their full life circle and that icludes breeding and breeding means: preserving, by producing an next generation).
But this means: Water changing and defeating one’s tendency to laziness is the most important thing for successful Paro aquaristics. Mind: Very small tanks could be decorated very differently and attractively, and they are much easier to handle as bigger ones. And you must handle them often if you have decided for these fish.