any aquarists here? I need some help...

[deranged chipmunk]

I have a 10 gallon tank, about 2 years old.
I will admit that I got the tank with very little knowledge of the biological filter, and how to maintain it.
I will also admit that I didn't get very good advice at the lfs, so unfortunatley have lost a couple fish along the way....

Up until recently I had 5 glowlight tetras (down to 4):
Hemigrammus erythrozonus - Wikipedia, the free encyclopedia

3 zebra snails:
Neritina natalensis - Wikipedia, the free encyclopedia

1 albino pleco (about 4 inches long):
Albino Plecostomus colorful tropical fish pictures

1 japanese algae eating shrimp:
Petshrimp.com -- All about shrimp

3 Amazon Sword tail plants:
Aquatic Plants for Freshwater Aquariums: Amazon Sword Aquarium Plant

I realize I'm overstocked (now), and never realized how large the pleco would get ! He was a little one of about 1.5 inches when I got him...

So here comes my question:
One of the glowlight tetras died last week, I replace it with another which died one day later. I haven't replaced it yet, do I need to? The local lfs said I need 5 tetras for them to feel safe and secure.
I tested my water:
Zero Ammonia, Zero Nitrites, and 40 ppm of Nitrate. I have done 25% water changes every day and still can't get my nitrates down.
PH is 8....

My other question:
My plants are dying! did really well for a couple years, but are now suddenly dying off... Any connection? I recently started adding CO2 to the tank (per the advice of lfs), should I fertilize as well?

I'm stuck on what I should do to proceed...
Any help or advice, etc. is very much welcome!

Oh, and here's a photo of the tank:


Here's a picture of the plants:


Also should add, I have an aqua clear filter, 10-20. And a bubbler
(as you can see in the pics)

Thanks guys!

ok, so, as far as the tetras go, they would feel more safe and secure that larger the school is. i always keeps schools of 6 or more. do they absolutely need to be in a school? no. plenty of people keep them in pairs and singly and those fish live a nice, happy, healthy life with no problems.

as far as the pleco goes, you can always re-home him and pic up a bristlenose pleco. they only grow to about 4' or so. or some otocinclus cats. they only grow to about 2". of course, you could also add a cory cat or two. all options for down the road, obviously.

as far as your nitrate issue, try rinsing out your media in some old tank water. or some dechlor water. that should get your nitrates down. not that they are really excessive for a planted tank.

now as far as the plants go. i need to ask a couple questions. do you fertilize the tank? how long have you had the airstone in there? what kind of co2 are you adding? and i hate to sound mean, but if those are the same plants that you have had for a year or so, they are just surviving, not thriving or doing ok. lol they should be HUGE. i have had amazon swords grow from 4 leaf plantlets to 2 foot tall 2 foot wide monsters within 2 months.....lol