i think everyone here has great points in their suggestions, I will say this personally i have never had luck with the bio wheels, the motors just never seemed strong enough and after a few days of running even in a new setup the bio wheel itself would stop spinning, which lead me to my decision for my smaller tanks to stick with the aqua clear h/o filters and fluval and penn plax canisters for my larger tanks. to me it just makes more sense to have more water running through more media, but then again that's my personal pref, now as to the massive water change, it has been my experience when things are way way out of wack, like 0 nitrates, and super high ph, to do a 50% water change what this does is removes enough of the water to drop the ph to a steady and safe level, along with cutting off the bio cycle and simply starting fresh with the cycle, it's basically like seeding a brand new tank, it will take a little while for the tank to re-cycle, however you still haven't removed too much of the beneficial bacteria, which brings the cycle to starting over again in the tank, it's a small tank and there aren't too many fish in there just as you do your water change make sure the temp of the water you are adding is very close to the temp of the tank water, you don't want to shock your fish with too cold or too hot of water. usually after doing this it takes a week for the tank to finish it's cycle and from there you shouldn't have any more issues. this is all from my own personal experience and experimentation. good luck,
Brandon

Your reading were a bit weird but not bad actually. I wouldn't try to read too much into 2 tetras dying, because it was probably the high pH or a pH swing that caused their deaths, not Nitrites, nitrates, or ammonia.

Tetras will die from the slightest little thing.

If your tank has pH of 6.8-7.2, 0 ammonia, close to 0 nitrites, and < 10-20 nitrates your tank is probably doing fine and you could start relaxing.

If your readings of the above substances are zero and nitrates are < 10 I personally wouldn't touch it anymore for like a week.

I believe tetras ideally need water similar to diskus, which would be soft acidic water. If you can maintain pH of 7 they should be fine though. You have other fish to considder, so I would shoot for pH 7.0 and unless waste products are building up cut way back on the water changes.

I do it no more than once every 2 weeks.

Ideally, the only waste product you need to dilute down with water changes should be nitrates if the cycle is working properly.

With your knowledge on proper feeding and the fact that you don't over stock your tank you shouldn't have any problem with maintaining that

I did as some suggested and changed the water, about 40%, with spring water, labeled "Origin: Protected California Mountain Spring", and this morning my numbers were:

pH: 7.8 (finally, something below 8.0ppm)
Ammonia: .25ppm
Nitrites: .25-1.0ppm
Nitrates: 0ppm

And there was a dead Guppy this ,morning. Kind of expected that.

Zambize

I have not tested the pH of my spring water. I don't have any left either, so I'll have to wait on that. The remaining fish seem quite normal, but hopefully they are happier/healthier with the lower pH. Now if it'll just stay that way.

I thought I might lose a fish or two, but it was "just" a Guppy. Although, he was cool. He was bright iridescent blue and silver, with an orange tail that had black spots all over it. For a Guppy, he was pretty cool. I think Guppies are highly underrated! Three fish gone this week, soon I won't have any!

Anyone ever lose many fish at one time?

Zambize

zambize, this is a very important time to stay on top of water tests, as your trying something new for yourself (and relatively rare in the forums). I would defintely test the spring water by itself, in the process discussed by miami with the air stone, so that you know where your target ph is, otherwise you don't know what ph you're working towards. It seems the ph is somewhere around 7.8 but you won't know until you measure the water alone.

Also, when i had very low ph (below 6) my water changes would show a 6.8 ph a few hours after, but then the very next morning my ph would drop below 6 again, until i found out my wood was the culprit. As soon as i took the wood out my ph is verrry stable at 7.4, like my tap water. So although you hit your target today, make sure to check tomorrow and the next day.

I personally record all my results (on a digital calendar on my comp) whenever i lay my hands on the tank, whether its water results or how much water ive changed or when ive changed my filters or done anything drastic. This helps me visualize when problems come up, so i can see the timeline, and so i can quickly achieve my targets.

Anyway, sorry bout guppy but glad to hear your ph is lower.... i think the death has skewed your amm and nitrite readings, but the water change should have kicked them to zero. let us know what results you get tomorrow.