I am sorry to hear about your last guppy. This is what I would do.
1. Scrap algae if any on glass.
2. Gravel/siphon any gunk in the water.
3. As you are doing step 2, remove 30% of the water (3 gallons).
4. Prepare your replacement water from your tap, the temp needs to match your tank water 78°. Add 1/2 cap of Seachem Prime, each line inside the cap are measurements. Then add back to your tank.
5. Add 2 teaspoons or 10 ml of the Tetra Cleaning Bacteria. Shake bag first.
6. Test your water tomorrow.
Now there is a difference between a water changes and tank cleaning.
Water changes I do weekly 25%. I vacuum. I prep my new water with a small amount of Seachem Prime. And that's it.
Everything thing else is a cleaning. The Tetra Cleaning Bacteria is to be only use monthly when you clean. Be cautious not to remove your natural beneficial bacteria.
There is another problem with your water. Your GH is too low. That means you have very soft water and are lack necessary minerals. GH and KH are recorded in ppm or degrees. Your GH is 0-30 ppm or 0-4 dGH (degrees general hardness) Guppies don't like this. They like a dGH of 8-20 dGH (140-320 ppm). Your KH at 120 ppm is 6-7 dKH. Because you have a tropical, planted tank with shrimp, the general guidelines for KH is 65-135 ppm and a GH of 65-135 ppm. I am going to try to link you some articles that may help.
Algae.
By the way guppies like to eat algae.
GH and KH
Water Changes
Tank Cleaning
Tank Cleaning with algae
Guppy Care Guide
I don't know what type of guppies you had but will give an idea. Guppies need high quality food. I am attaching a photo of my list of foods that I use.
These should help. Be careful to use store products to control algae products, it could kill you shrimp.
If you have any questions. I will here.
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