HELP! My Adult Shrimp Have All Died!

Here's my info:

Tank

What is the water volume of the tank? 10 gallons
How long has the tank been running? 7 months
Does it have a filter? Yes - Hang on Back with Pre-Filter Sponge
Does it have a heater? Yes
What is the water temperature? 75 degrees Fahrenheit
What is the entire stocking of this tank? (Please list all fish and inverts.) Approx. 20 freshwater shrimp fry Approx. 10 freshwater shrimp Juveniles 3 Mystery Snails 2 Nerite Snails

Maintenance
How often do you change the water? Weekly
How much of the water do you change? 30%
What do you use to treat your water? Tetra Aquasafe Plus
Do you vacuum the substrate or just the water? Just the water

*Parameters - Very Important
Did you cycle your tank before adding fish? Yes
What do you use to test the water? API Master Kit
What are your parameters? We need to know the exact numbers, not just “fine” or “safe”.

Ammonia: 0
Nitrite: 0
Nitrate: 20ppm
pH: 7.6

Feeding
How often do you feed your fish? Every other day
How much do you feed your fish? Whatever they can eat in 30 minutes
What brand of food do you feed your fish? Aquatic Blended Foods Super Veggie Sticks & an Algae Lollie Once a Week
Do you feed frozen or freeze-dried foods? Frozen Snello (Snail Jello)

Illness & Symptoms
How long have you had this shrimp? 5 months
How long ago did you first notice these symptoms? A month ago
In a few words, can you explain the symptoms? I see no real symptoms, I just slowly found my adult shrimp dead, one by one, over the past month.
Have you started any treatment for the illness? No
Was your shrimp physically ill or injured upon purchase? Not that I was aware of
How has its behavior and appearance changed, if at all? I noticed no odd behavior prior to the deaths of my adult shrimp.

Starting a month ago, I began losing my adult shrimp one by one. It didn't happen quickly, maybe one dead shrimp every 3 or 4 days. I saw no change in behavior at all. The only odd thing I noticed was in ONE blue shrimp - he faded & spotted over the past month & he died today. I was doing extra water changes because my Nitrates were high. I now have my nitrates down to 20ppm, but I lost my last adult today. I've now lost all my adult shrimp and have only fry and juveniles. I'm worried that whatever is wrong will keep them from growing to adulthood and through adulthood.

 

What were your nitraits before you got them down to 20 and how long were they high?
In these situations photos speak tons, do you have any to see if any disease can be seen before they died

 

Hello - my Nitrates were pretty high, around 40ppm & it took me two months of doing water changes twice a week to get it down to 20ppm. I also started feeding them less/removing the food after 30 minutes.
I didn't notice anything odd EXCEPT for the blue shrimp that just died. I noticed a few weeks ago that he'd faded & had dark blue spots on him. Here's the best picture I have of the blue one that died today. You can see the dark spots on his side - he was once that entire color.
I haven't noticed this in the other adult shrimp that died, though.

 

40ppm would be toxic to shrimp that alone would cause death and im not sure if I can see rustspot on the one in the photo, I'm going to tag you onto Crimson_687 for you to read what I've put there. Both posts are for the same problem

 

40ppm would be toxic to shrimp that alone would cause death and im not sure if I can see rustspot on the one in the photo, I'm going to tag you onto Crimson_687 for you to read what I've put there. Both posts are for the same problem

Thank you!!!