Fancy Guppy struggling to swim and fish death

Our oldest guppy we've had about 6 months has always been a little odd shaped but as of recently he's begun looking alot worse. I'm not sure exactly what it is but he's lost alot of his side coloring, sits at the bottom of the tank frequently, and just looks exhausted every time he swims , it looks like it requires massive effort to move his tail and his entire body goes back and forth when he's swimming. Any ideas?
We have also over the last 2 months had 4 out of the blue fish deaths- 3 of which were fancy guppies that I attributed to the filter being too strong as I found them jammed between it and the wall so we shifted to 2 -10 gallon filters instead of the 20 gallon one we had. We then lost our oldest molly the other day. All of these fish were swimming around fine and acting totally normal until they were found floating. My tank is a little cloudy right now due to just changing the water but is usually crystal clear and will clear up again by tomorrow.

Ammonia between .25-and .5 ppm which is where it always seems to stay no matter how many changes i do. I do treat with Prime conditioner ammonia, nitrate, and nitrite detoxifier with weekly water changes.
Nitrates 5ppm- I've been doing 25% water changes for 3 days since we lost out molly
Nitrites 0ppm
PH steady between 6.8 and 7

Oh and 20 gallon tank with max 2 mollies, 3 guppies, 1 platy.

What is the GH? Your fish all need hard water.
The ammonia is a problem. What filter do you have and how do you clean it? Do you have substrate?
The guppy looks to have a curved spine and is quite thin. There are a number of possible causes (genetic defect, injury, internal parasites, TB).

The positive ammonia reading will be causing all of your issues. You should do a 100% water change every time that you see the presence of ammonia as a matter of priority.

It could be that bits of the deceased fish are rotting within the filter system which may explain the consistent positive ammonia reading. Do a thorough filtration media clean in a bucket of the aquariums water.

Hi and welcome to the forum

I would find out the GH (general hardness) of your tap water, and get your tank and tap water tested for ammonia, nitrite and nitrate, by a pet shop. Take your test kits to the pet shop and test the same water when the shop does. You can compare the results from your test kit to theirs. If their test kit says the water contains 0ppm ammonia and your kits says there is some ammonia, then you have a faulty kit.

You can normally find the GH (general hardness) and KH (carbonate hardness) of your water supply on your water supply company's website or by telephoning them. If they can't help you, take a glass full of tap water to the local pet shop and get them to test it for you. Write the results down (in numbers) when they do the tests. And ask them what the results are in (eg: ppm, dGH, or something else).

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I would say the problem is low GH and low pH. Mollies, guppies, platies and swordtails don't like soft water or acid water (pH below 7.0). If you have soft acid water, it will knock them about pretty badly.

You want the GH around 200ppm for guppies, platies and swordtails, and about 250ppm for mollies. The pH should be above 7.0 for all of them.

Your pH isn't too bad, but if you have soft water, then a pH of 6.8 combined with the soft water will do some damage.