Hardy Fish?

[Hathaway]

I read quite often that some fish are not hardy and therefore should not be put in a new tank. However when the term "new tank" is used, does it refer to one which has literally just been set up, so would therefore be a completely inappropriate fish for a fish in cycle? Or does it also include tanks which have gone through a fishless cycle and are ready for stocking?

The reason I ask is because I've also read that the least aggressive fish should be introduced first to minimise territorial behaviour on new fish. Incidentally it seems my choice of fish includes my semi-aggressive fish (barbs and RTBS) being the hardiest and vice versa. Though I am in the process of fishless cycling my tank and won't be adding anything until that is complete.

 

[the_lock_man]

It includes newly-cycled tanks.
 
An established tank has built up its "biofilm", protecting the filtration bacteria. Generally, established tanks are around 6 months old.

 

[Hathaway]

So am I better off putting in hardier but more "aggressive" fish in first, and then more peaceful but less hardy fish in after?
I'm fast learning this hobby is a statistics game lol.

 

[AmtotheBurr]

It used to be suggested that you could remove the established fish, add the new fish, rearrange the tank, add the original fish back in, etc. to help with dominance. I don't know if that same suggestion is used anymore.
You'd want to quarantine any new fish before dumping them in an established tank, so keep that in mind when you decide how you're going to do this.

 

[Byron]

So am I better off putting in hardier but more "aggressive" fish in first, and then more peaceful but less hardy fish in after?
I'm fast learning this hobby is a statistics game lol.

 
No, this is somewhat reversed, and a bit mis-understood.  I'll try to explain.
 
First, to the initial question about "established," as lockman said, this refers to a tank where the biological system has pretty much stabilized.  A tank goes through the cycling initially, when the nitrifying bacteria are introduced and begin multiplying.  But this is only the beginning...over a period of a few months, usually, the other bacteria appear (most of these live in the substrate, some in the filter, and some everywhere on any surface) and the biological system settles.
 
Some fish are more sensitive than others to the water around them, and these are the ones that should only be introduced to an established tank.  It has nothing to do with cycling...no fish should be subjected to "cycling" per say.  But once the tank is cycled, there are some fish that will be OK, and some that will not settle in.  I hope that is clear, ask if not.
 
To the aggressive part of the question...generally speaking, aggressive fish should be added last.  This is because they are then being introduced to an aquarium containing other fish that are likely "settled," and the introduction of somewhat aggressive fish is less likely to impact the other fish.  The aggressive fish have to "fit in," as it were, to what is already there, and generally this works better than the reverse.  However, there are in nature always exceptions.  And sometimes as Amber mentioned, re-arranging the tank aquascape can help, but this generally works when new fish are added to an existing group of the same species; the goal is to break up the established territories by re-arranging wood, rock, plants, so the existing fish are not "at home," in a sense, when a rival is introduced.  This sometimes works, sometimes not.
 
If we knew your intended fish, we would liekly be able to suggest which to add first, second, last, etc.  Or which to avoid altogether.  "Aggressive" fish can sometimes literally destroy an existing tank, and by this I mean the health of the existing fish is jeopardized.
 
Byron.

 

[Hathaway]

The fish I've settled upon are Tiger/Green Tiger barbs, a single Red Tailed Black Shark, German/Electric Blue Rams and Cardinal Tetra (really not keen on changing this to Neons even though I know they're considered easier). I may consider Clown Loaches with the idea of rehoming them after they grow to a certain degree - I know some people have strong views on this but I've yet to decide.

But with the barbs and shark being the most territorial I would have considered placing them in last, however most sources confirm they are rather hardy fish. Whereas the Cardinal Tetra and Clown loaches are not particularly hardy fish but very peaceful. Rams I get conflicting information on in terms of hardiness, but seem to be friendly unless spawning.

 

[Byron]

OK, there are some very significant problems with the mentioned fish in the same tank.  It involves several aspects, not just aggressive behaviours, but activity, and water parameters.  I also do not know the tank size, so on this I will just set out specifics for the species.  Here goes.
 
First, Tiger Barbs are naturally "aggressive" in nature.  They must be kept in a larger shoal (group) than some other shoaling fish might need, and their tankmates have to be carefully considered to exclude any sedate fish or those with long fins.  A group of ten minimum, but a few more is better, and in no less than a 30g (113 litre) tank; the regular and the green variety can be combined in this group, as they are the same species (the green is a man-made variant).  This tank would house the Tigers on their own, in a group of 10-15.  Substrate fish that are fairly "tough" might work with them, such as some of the loaches (tank size will affect this, as loaches can get largish and need a group too), but no upper fish.  If the tank is larger, and here I would say 50 gallons and up, then you can add some other upper level species, but staying with the barb family or perhaps the danios.  A few characin (tetras) species might work, depending.
 
The Red Tailed Shark, Epalzeorhynchos bicolor, is not really a community fish.  This fish attains five inches, and it needs at least a 4-foot (120 cm) length tank so it will have the room it expects.  It is very aggressive with its own species (it probably lived in solitude except when breeding) and as it matures is often aggressive with other fish especially those resembling it and those with vertical stripes [= your Tiger Barbs]. Should be kept solitary (one fish per tank) with carefully-selected tankmates like the larger barbs and rasbora.  Bottom fish (loaches and most catfish) should not be included with this species.  [Just a side-note, as I said it "probably lived in solitude:" this fish was believed to be extinct in the wild, and only aquarium raised fish are seen.  Only this year, or last, it was discovered but so far it has not been observed sufficient to ascertain some of its behaviours/requirements in the wild.]
 
The blue rams and cardinal tetra are good tankmates, as they occur in near-identical parameters.  However, the rams need warmth, no less than 80-82F/27-28C, and this will not suit some other species, such as the Tiger Barbs and loaches, but there are many others that are healthier at more "normal" temps for tropicals around 25-26C/76-78F.  I know this may not seem like much, a couple degrees, but to the fish whose physiology is governed and driven by temperature, it makes a very big difference and can seriously impact on their health.
 
The other parameters for rams somewhat depends upon their origin; interestingly, this is a species that seems to do best (meaning, it may live its normal lifespan of around four years) when maintained in water having the same parameters as those in which it was bred/raised.  The cardinal tetra is a very soft water fish, and will not live its full lifespan (ten years and over) if kept in harder water.
 
Last on the clown loach...a shoaling fish needing at least five (sometimes four will work, but not always), and a large tank of at least six feet but most reliable sources suggest 7-8 feet.  It will grow to around 8 inches (20 cm) fairly quickly, then more slowly up to 12 inches (30 cm)average, though there are confirmed reports of fish attaining 16 inches (40 cm).  It is a highly social fish, as are most all of the Botine loaches, and without the group its very aggressive traits will exert themselves resulting usually in dead subordinate fish.  As for acquiring this fish with the intention of re-homing it later due to size, this is a practice I cannot endorse.  I appreciate it may not seem an issue, but again we are dealing with living creatures that have evolved according to natural rules and it is cruel to the fish to force them into situations they are not prepared for, and large space in which to grow is one of these.  Aside from the problems this can cause a growing fish, physically and mentally, many of us feel that when we acquire a fish we are providing it with its final home on this earth, and this is the attitude that will ensure we give the fish what it expects and needs.  The fish's needs should be primary, and our wants forgotten when the fish's well-being are at stake.
 
Feel free to question anything I have not made clear.  That is the great benefit of a forum, we can all learn before we err with the fish.
 
Byron.