Nitrites & nitrates too high???

First topic message reminder :

I have a 20 gallon, started in late November, with 12 glo danios and two small (less than 3cm each) plecos.  All along it has been my "perfect" tank, crystal clear, no problems with fish or anything else that I was aware of.  It is heated to 79 degrees (f), has a HOB bio filter for 20 - 55 gal tanks, has two bubblers and also has a 5 watt UV light.  
I do 30 - 40% water changes weekly, have well water, slightly hard.  I have been testing with the 5 in 1 strips and things looked good, no nitrites, very, very slight nitrates, everything else where it should be according to the strips.  Suddenly, this week's test showed nitrites off the chart and nitrates about 40.  Repeat testing didn't change so it wasn't a bad strip.
I dumped in a bottle of tetra safe start, supposed to lower nitrites AND nitrates, amount for about 15 gallons which was all I had on hand, no change at all 24 hours later.
Next, I did a 40% water change, very slight change in nitrites.  Fish appear fine, eating well and looking good, but were more active after the water change, (may be normal with ANY water change?)
I am NOT overfeeding, in fact, I wonder if I am feed them enough.  I also switched filters going to a bigger one a few weeks back.  
I also treated the tank with two doses of Malachite green and formalin as a precaution because of a severe outbreak of ich in another identical tank housing 15 glo tetras and also two tiny plecos about three weeks ago.   Those fish recovered with only one loss, and that tank tests ideal  with no nitrites, slight nitrates.
I just ordered the API test kit with the vials since I hear that strips are not really accurate but it won't be here until Monday.
What in the world is going on here and should I be worried????

Well, the new test kit came. I still find it hard to decide what color is what color since one is a transparent liquid and the other is a color pasted on white paper. But, I was encouraged to find the Nitrites at 1. ppm, the ammonia less than .25. Nitrates were still up but down to 20. I guess that is better????
I tested the other 20 gal started at the same time and it has 0 ammonia and 0 nitrites and 10 nitrates, which is good, I think. Both had 7.4 for ph.

as long as nitrates are below 40 its usually considered OK. however 1 ppm Nitrite (really anything above 0 ppm) is considered dangerous.

I still did a 40% water change tonight. Better than it was since it was off the top of my strip test, or somewhere around 10ppm.
Hopefully a few more daily water changes and it will settle down.
Really frustrating!!!! I'm going to try to find some more plants but the lfs was pretty much sold out of low light ones.
Do you think a Pothos will help? If I do that, I just rinse off the soil and put the roots in and leave the plant itself out, right?

yup, it does take a little while before the pothos changes its roots to a submerged one. So it will take a few weeks before it really takes off.

Also keep in mind that the higher the Ph, the more dangerous ammonia and nitrite are. A mid 7's ph and nitrite of 1 can be lethal to fish. Up the volume of your w/c to 75% per.