The 50% Rule

[Kaidonni]

Having decided I will not be rehoming my White Cloud Mountain Minnows at this time, I need to up their numbers from two males and one female to two males and four females. If I follow the 50% rule, this means 1.5 fish, which leaves me with two options: a) play it safe and get only one female this time around; or b) fudge it to 66% and get two females this time around. I'm already moving my weekly filter maintenance and 50% water change forward one day and conducting it prior to the addition of any new fish so that they aren't stressed by a large water change on top of transportation, acclimatisation and getting used to a new habitat, but can I get away with getting two Minnows rather than one, and slightly side-stepping the 50% rule?
 
In addition, what is the advice on fish shop water from the bag going into the tank? Obviously I need to siphon the water from the bag and replace it with tank water to acclimate the new fish before adding them to the tank, but seeing as I will have already conducted my water change prior to this, any fish shop water that finds its way into the tank won't be dealt with until next weekend. I'm not the most confident person when it comes to using nets... I have considered using a small plastic water jug in place of the bag while acclimating the new fish to my tank's water - that way I can gauge how many ml of fish shop water is left before I start adding my tank's water to the mix - but the net option is still on the table. Would 100-200ml be too much fish shop water to introduce to my tank?

 

[Munroco]

I really don't understand these numbers you are using. If you want 6 mountain minnows, as long as you have the space then buy another 3 or am I Missing something. I think your complicating things unnecessarily for acclimatising fish. I just float the bag to equalise temperature, cut it open, let some tank water into the bag then release the fish. I should probably quarantine them for a week or so but I don't have the space or patience. Unless either you or the LFS are dosing your tanks with something, the water should have similar properties.

 

[StevenF]

You might want to explain this 50% rule.  I have not heard of it or you are misinterpreting it.  Also it would be helpful to know the size of your tank and a list of all fish in the tank.

 

[Kaidonni]

I have seen it advised that, when adding additional fish to the aquarium, only 50% of the present stocking level should be added at a time to avoid over-taxing the beneficial bacteria in the filter. In other words, if I had a shoal of ten Neon Tetras and I wanted another ten (I don't, but it's just an example), I'd only add five the first time around, then the other five later on - not all ten at once.
 
As to the tank, at it's present level it is in the range of 45-50 litres, and there are only two male and one female White Cloud Mountain Minnows. Live plants.
 
As to acclimatising the new fish, I just play it safe. I know they are WCMMs and are very hardy, so I'm probably worrying more than I need to. I do use Prime in water changes and dose with Seachem Flourish Comprehensive, although that is probably completely irrelevant to acclimatising them and won't have any impact. I do however prefer to avoid too much water from the bag getting into the tank when I release the new fish, so changing the mix as much as I can prior to that is preferable as using a net on the bag at that stage would be awkward, so water from the fish shop will likely find its way into the tank.

 

[Byron]

I have seen it advised that, when adding additional fish to the aquarium, only 50% of the present stocking level should be added at a time to avoid over-taxing the beneficial bacteria in the filter. In other words, if I had a shoal of ten Neon Tetras and I wanted another ten (I don't, but it's just an example), I'd only add five the first time around, then the other five later on - not all ten at once.
 

 
As so often happens in this hobby, people (especially beginners) want "rules" for everything; this is not really bad, because it at least means they care and want to do things right.  But unfortunately, there are few relevant rules.  While it is possible to over-stock to the extent of a mini-cycle, it is not always going to occur, and other factors play into it.
 
As a tank matures, it builds up a host of bacteria everywhere.  Here we are dealing with nitrifying bacteria, and these can multiply relatively quickly once they are present, to deal with increases in ammonia and then nitrite.  This is quite different from a new tank with no living bacteria, and fish being suddenly added.
 
You mention live plants: with these, it is next to impossible to ever add too many fish to cause ammonia/nitrite issues.  Provided you are not way overboard obviously, but you could easily quadruple the present fish and not have any issues with ammonia or nitrite, with live plants if they include some relatively fast-growers (floating plants are ideal for this, they are literally ammonia sponges).
 
Second point to make is that shoaling fish should be added together if at all possible.  Here, I don't mean get rid of the three and buy six new ones or something, but if adding to an existing group of shoaling fish, add all intended new fish at the same time.  For one thing, this is easier on the new fish, as the more there are, the faster they will settle, and this means less stress which means less opportunity for disease.  It is not so critical with this species, but with some, you also want to establish the natural hierarchy at the beginning, and not be adding to the group later.  This is why angelfish must be added together, never bit by bit.  Much less of an issue with white clouds.
 

As to acclimatising the new fish, I just play it safe. I know they are WCMMs and are very hardy, so I'm probably worrying more than I need to. I do use Prime in water changes and dose with Seachem Flourish Comprehensive, although that is probably completely irrelevant to acclimatising them and won't have any impact. I do however prefer to avoid too much water from the bag getting into the tank when I release the new fish, so changing the mix as much as I can prior to that is preferable as using a net on the bag at that stage would be awkward, so water from the fish shop will likely find its way into the tank.
 

 

In addition, what is the advice on fish shop water from the bag going into the tank? Obviously I need to siphon the water from the bag and replace it with tank water to acclimate the new fish before adding them to the tank, but seeing as I will have already conducted my water change prior to this, any fish shop water that finds its way into the tank won't be dealt with until next weekend. I'm not the most confident person when it comes to using nets... I have considered using a small plastic water jug in place of the bag while acclimating the new fish to my tank's water - that way I can gauge how many ml of fish shop water is left before I start adding my tank's water to the mix - but the net option is still on the table. Would 100-200ml be too much fish shop water to introduce to my tank?
 

 
These two cites deal with the same issue.  Never, never introduce fish store water from the bag into an aquarium, be it the QT or the display tank.  There could be all sorts of pathogens, and while some if present may get in on the fish, others will not.  A QT for new fish is a wise investment these days; I have twice lost about half my display tank due to not quarantining or not long enough.  Hard lesson learned.  Ich and such issues is fairly easy to ID and deal with, but there are internal protozoan (that seem to be very prevalent in commercially-raised fish these days) that are impossible to detect until the fish dies.  But QT or not, do not get the bag water in the tank.
 
The bag should be floated in the aquarium (to equalize temperature).  Then you can add water from the aquarium to the bag (sometimes I remove a bit of the bag water before floating, if it is a lot) slowly, or do this by the drip method.  The aim is to have the water in the bag when finished at a ratio of 1/4 original water and 3/4 tank water.  Then net the fish out of the bag into the tank.
 
Byron.