What Species Is This?

If you are thinking of discus [I assume this is why you want the highest temp and low pH???] in the 270 litre (70 gallon), I would not put any of the named tetra in with them.  Not only are they not suitable temperature-wise, but these species tend to be a bit feisty (some more than others) and with sedate fish like discus this usually means fin nipping and active swimming which will stress the discus.
 
On lowering the pH...this will only be successful if the GH and KH are low, as the three are closely related.  You can ascertain the GH and KH of your tap water from your water authority, likely on their website.  GH refers to general hardness, and KH to carbonate hardness or Alkalinity.  If these are high, the pH will not lower because the KH especially acts as a buffer to prevent pH fluctuations.  I went into this in detail in another thread earlier this week, here:
http/www.fishforums.net/index.php?/topic/439302-guppies-and-their-water-general-hardness-and-alkalinity-question/
 
To the initial issue of increasing the numbers of the tetra you now have together.  This assortment is in a 100 litre tank, at 84 degrees.  First, this temperature is way too high for any of these fish.  Temperature for tropical fish is extremely important, as the temperature significantly affects the physiological functions.  In all fish, the amount of energy needed to maintain the physical homeostasis increases the warmer the temperature, and this has the effect of literally burning out the fish.  Stress also results, which further weakens them.
 
If these fish are intended for the larger tank, without discus mind you, then you could increase the numbers of all of them to provide a better environment.  I don't know how long these fish have been together (before you got them), in the present small tank, and this is important because the stress from insufficient numbers plus the small space may have already affected them, and this is usually irreversible.  When I say small space, the 100 litres is not too small for this number of fish in itself, but given that each species needs more, it would be too small to properly house them.  Moving such weakened fish to a larger tank and adding more of the species can sometimes cause even more problems.
 
Feel free to ask any questions on any of the above and I will try to explain more.
 
Byron.

Byron said:

If you are thinking of discus [I assume this is why you want the highest temp and low pH???] in the 270 litre (70 gallon), I would not put any of the named tetra in with them.  Not only are they not suitable temperature-wise, but these species tend to be a bit feisty (some more than others) and with sedate fish like discus this usually means fin nipping and active swimming which will stress the discus.
 
On lowering the pH...this will only be successful if the GH and KH are low, as the three are closely related.  You can ascertain the GH and KH of your tap water from your water authority, likely on their website.  GH refers to general hardness, and KH to carbonate hardness or Alkalinity.  If these are high, the pH will not lower because the KH especially acts as a buffer to prevent pH fluctuations.  I went into this in detail in another thread earlier this week, here:
http/www.fishforums.net/index.php?/topic/439302-guppies-and-their-water-general-hardness-and-alkalinity-question/
 
To the initial issue of increasing the numbers of the tetra you now have together.  This assortment is in a 100 litre tank, at 84 degrees.  First, this temperature is way too high for any of these fish.  Temperature for tropical fish is extremely important, as the temperature significantly affects the physiological functions.  In all fish, the amount of energy needed to maintain the physical homeostasis increases the warmer the temperature, and this has the effect of literally burning out the fish.  Stress also results, which further weakens them.
 
If these fish are intended for the larger tank, without discus mind you, then you could increase the numbers of all of them to provide a better environment.  I don't know how long these fish have been together (before you got them), in the present small tank, and this is important because the stress from insufficient numbers plus the small space may have already affected them, and this is usually irreversible.  When I say small space, the 100 litres is not too small for this number of fish in itself, but given that each species needs more, it would be too small to properly house them.  Moving such weakened fish to a larger tank and adding more of the species can sometimes cause even more problems.
 
Feel free to ask any questions on any of the above and I will try to explain more.
 
Byron.

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Thanks byron i found this very helpful, the fish in question have been together for a couple of years so i was told and they were in a 260L tank,the 100L is just one that i had running without fish in only a plant. The water has been tested and all is perfect other than ph, what temperature would you recommend for these tetras and the striped raphael catfish? I am soaking some mopani wood and researching some other methods of regulating ph.

Far_King said:

That's a shame but could be best for them in the long run.  Any idea what sort of water parameters they were living in before?  It's possible they may have adjusted.

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I know the temperature was around 80-82 before i got them and the water wasnt healthy (unfiltered for 2weeks+)

another pic

First, from the photo in post #18 the reddish tetras are Rosy Tetra, not Serpae.  This is good, as the Serpae is notorious for its fin nipping and feisty behaviours, and having less than 8-10 of them is almost always a recipe for disaster.  The Rosy are one of the most peaceful tetras, so you are better with these, whatever you do.  And re the discus again, a group of Rosy Tetra would be suitable, say 8-12 of them.  They are good tankmates for angelfish too, same reasons.  And obviously they can tolerate the warmer temperatures, though I wouldn't go above 82F.  I'll return to temp as I respond to your questions.
 

Thanks byron i found this very helpful, the fish in question have been together for a couple of years so i was told and they were in a 260L tank,the 100L is just one that i had running without fish in only a plant. The water has been tested and all is perfect other than ph, what temperature would you recommend for these tetras and the striped raphael catfish? I am soaking some mopani wood and researching some other methods of regulating ph.
 

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Given that these fish have been together for a couple of years, you might leave them as they are (the tetras I mean, I will come back to the Raphael), in the 100 litre tank.  Lower the temp to around 76-77F.  There should be no issues with the other parameters, as they have likely become accustomed to them.  This is one of those situations where long-term inadequate housing (by the predecessor aquarist) may well have affected the fish, or not, we cannot say; individual fish sometimes act outside the norm for the species.  Unless issues among them develop (thinking aggression here) they might be fine as is.  However, you could also move them to the larger tank, and increase their numbers.  Keep in mind though that discus are not suitable with most of these.
 
The Raphael catfish has quite a range in temperature, from 73 to 86F, roughly speaking.  However, as with most fish, it is best to maintain them long-term around the middle of their given range.  The extremes (high or low) are what they can tolerate, but should not generally be kept at for extended periods.  Same reasoning as I mentioned previously, concerning the internal physiology of the fish and the effect temperature has on their proper functioning of the various biological processes.  However, this fish needs the larger tank, as it will attain six inches minimum, and up to eight inches.  Keep in mind too that it will eat a lot, and thus excrete a lot, which affects water quality.  And it may decide to rearrange the aquascape to its own liking.  It is also possible that it will eat small fish as it matures.  They are truly greedy when it comes to food, and it is easy to overfeed them (they will simply vacuum up everything in their path) which causes severe internal problems and can lead to disease and death.
 
To the pH, as I noted in the other thread if you read it, you must first ascertain the GH and KH, as attempts to adjust pH will usually fail without regard being given to the GH/KH.  Lowering pH with organics, such as wood, peat, dried leaves, will work, but only if the GH/KH are fairly low.  If these are moderately high or higher, the pH will not lower, and if you persist and cause fluctuating pH levels back and forth, the fish will have a far worse time of it.  Fish must regulate the pH of their blood to equal that of the water, so this again causes severe stress and uses valuable energy that weakens their immune systems and other issues.  Contact your municipal water authority, maybe their website, and get the GH and KH numbers.  Only this will tell you and us what may (or may not) be possible.
 
Byron.

I had the water tested and i was told other than ph being high it is as good as it gets. The raphael catfish is pretty old i think its probably 5-6yrs+. He had some kind of skin problem when he came to me but i think that having the temp high for a week has cleared that up, he seems alot more active each day too and his colours/stripe is more vivid and hes hungrier, im lowering the temp one degree per day at the moment following your advice and preparing the larger tank which is going to be a sandy bottom for the benefit of the catfish.

I had the water tested and i was told other than ph being high it is as good as it gets.
 

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Without the numbers, none of us knows what this might mean.  Check your municipal water site.

Byron said:

 
Without the numbers, none of us knows what this might mean.  Check your municipal water site.

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I will find out exact within next few days and reply here