Tetra lays on bottom

Question
Hi Christy,
Thank you for taking my question. My tetra (I do not know his specific type- he's about 1-inch long and has nearly transparent tail end with a reddish coloring). he is laying on his side on the gravel for the majority of his time and then sporadically swims with great effort. He is eating. No signs of disease or fungus. He even pokes at other fish as he darts about the tank when he does swim.
It is an 18-gallon tank.
pH-I struggle to keep the pH between 7.0-8.0. It is very hard water.
GH-120
KH-120-240
NO2-0
NO3-80-160

My live plants have green algae on them. The other 4 fish seem fine (also tetra family) except we are fighting ich on one high fin black tetra. Mollies die right away. Algae eaters don't live very long.
I add treated water when it evaporates.
I change the carbon filter every week, except I don't use one when treating ich.
I add Nitraban 1x week. pH Down daily.

Answer
Hi Linda
First suggestion I have, is to cut out the ph down product.  Most of the fish available at the stores-including the ones you have will do just fine with a ph of 6.0-8.0 and a tad higher or lower then that even.  Pretty much the only exception to altering the ph, would be a fish that's wild caught, or a super sensitive fish like discus or German rams.  You really need to know what you're doing-and that involves the gh and kh(general hardness and temporary hardness) of the water, otherwise it just bounces back to where it was to start with.  Here's a link that explains it a bit more in detail of all that's involved with it:

http://www.drhelm.com/aquarium/chemistry.html

If your ph is way over 8.0, then you need to address the gh and kh issues to lower the ph level, that link has some good suggestions.  And, you'll need to make sure to adjust the ph in your new water you add to the tank so the ph doesn't get all out of whack when you add it to the tank.

Next, I'd cut out the Nitraban as well.  I believe that's one of those products that supposedly adds bacteria to the tank(?), if your tank is cycled, which it looks like it is form your figures you posted, no need for it.  And, most of those products don't work anyway....so save your money :)  The only "chemical" product that's needed for a tank, is a water dechlorinator that removes both chlorine and chloramine(since water companies will use both at different times of the year sometimes) for water changes.

Now, to your test levels. I don't see a reading for ammonia, so I'd recommend picking up a test kit also that tests for that.  The 3 things you mainly want to watch are the ammonia, nitrite(no2), and nitrates(no3), and just monitor the ph levels periodically to make sure they're not fluctuating too badly.  
Nitrite level is good, you always want that and the ammonia to be at 0 ppm, but the nitrate(no3) is bad....That one you want to keep under 20 ppm ideally.  

Now, you mention that you add water when it evaporates, which is fine.  But....are you using that in place of weekly water changes?  I recommend for a normal stocked and fully cycled tank, 25% weekly water changes and a gravel vacuum done weekly as well(use the gravel vac to do the water changes).  Sorry if I"m misunderstanding you and thinking wrong, but if that is the case, that's why your nitrates are so high.  The weekly water changes helps to keep all those levels in check by removing those toxins(ammonia, nitrite, and nitrates) out of the water, along with the biological filtration(your beneficial bacteria which consumes the ammonia/nitrites, and the nitrates are the byproduct of that).  It's very possible this is what's causing the problem with the one tetra-he just may be more sensitive to it, and also the could be the problem with the one that has the ich.  It's also possible both those problems are from the ph adjusting up and down too rapidly from that ph down product.  

What I'd recommend at this point, do daily water changes of about 20-25% until you get that nitrate under 20 ppm.  Don't use that ph down or the other Nitraban either, just a water dechlorinator if you're on city water.  Don't do a large water change thinking that'll get the levels down quicker, because that can actually stress the fish out even more.  Best bet to do it slowly over a few days period.  Do a search online for "old tank syndrome" as well, that sounds kind of like what you're dealing with, especially with new fish dying soon after you get them, that's one of the classic signs of OTS.  Once you get the levels in check and where they should be, get on a weekly schedule of changing out 25% of the water with a good gravel vacuum.

For the carbon, I wouldn't use that anymore either.  If it's the type that comes in the filter pad, that's fine, but if you're adding it separately, again, save your money on that.  In 8 years of fish keeping, I've only used it a handful of times, and that's just to remove medications from the water.  It's not a necessity to use all the time in a filter.

One more thing, what are you using to treat for the ich problem?  Because tetras are sensitive to a lot of the medications out there for ich, and also to salt if you're using that option for treatment.  Just be sure whatever you're using, you follow the "safe dosage" instructions for tetras and scaleless fish, and you should be fine. Here's a few good links I have saved on ich and different treatment options:
http://www.peteducation.com/article.cfm?cls=16&cat=1791&articleid=2421

http://en.wikipedia.org/wiki/Ichthyophthirius

http://www.aquahobby.com/articles/e_ich2.php

Give it some time, get everything straightened out in the tank, get the ich cleared up, and then add a few more fish.  Add them slowly, only a few at a time as well, then give it at least a week or so before adding a few more fish.  Hopefully getting the water parameters back in check, and not using those other products will help the tetra that's laying down.  If not, let me know.

Hope that helps, and let me know if you have any more questions!  Good luck!
Christy