Help with fin rot please

Question. We have a 455L ( about 120gallons )tank with some guppy's, cardinal tetra, corydora's, rasbora's, hillstream loaches, kuhli loaches and some neocaradina shrimp. The tank is 23℃ ( 73,4℉) ph is about 7,2 /the no³ is about 10 /and the no² is 0/ the amonia test is also 0. Every week we change about 10% ( 40L ) and every other week we do a filter clean. The problem is the guppy's and only the guppy's have fin rot on the tails. When we see the symptoms we put that guppy in quarantine ( treat it with sera omnipur for a few days and than 14 days in quarantine before it goes back ) and then a few weeks later an other guppy has it and so on. What would you guys do in this scenario? Because its now the 5th guppy in like 2 months. ( We don't think the water is bad because the parameters are good and the shrimp breed like mad every 14 days there is about 60 - 70 small ones in the filters.)

Is your water hard? Only other thing I can think of. I believe guppies need harder water.

Do more than.a.10% water change. Get that tank.water clean. This is your first line of defense when it comes to treating most anything in the tank. Fin.rot is one that benefits from essentially a water changeover, done over time. 120 gallons is a decent size tank,.but I'd.say.do some 30% changes for a bit, and get that water freshened up.

It does also take.a bit of time to heal fins. A few weeks to months depending on circumstances.

Hi and welcome to the forum

Are you sure it's fin rot and not other fish biting their tails?

Have you got any pictures of the fish with the problem?

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What is the GH (general hardness) and KH (carbonate hardness) of your water supply?
This information can usually be obtained from 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).

Guppies like a pH above 7.0 (yours is fine), and a GH about 200ppm.

Cardinal tetras and most rasboras come from water with a pH below 7.0 and a GH below100ppm. A pH of 7.2 is fine for these types of fish unless they are wild caught.

GH isn't likely to be the issue but it helps the fish if they have water that resembles what they evolved in.

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If you add some floating plants it might give the guppies more hiding places and encourage the other fish out into the open.

First of all. Thanks you all for the comments ( advice ) .
My kh is about 9° and gh is about 10°. ( We use sera test strips.)

We think its fin rot because edge of the tail isn't nice and smooth anymore and the edge is white with sometimes a little red. And if we wait a bit longer small pieces are hanging loose on the tail. This guppy's tail was nice and smooth not like this like there are pieces missing. And i know its really hard to see but you can see the white edge on his tail. ( The white spot in the middle of his tail he always had )

A month are so ago we did put some floating plants in some amazon frogbit and some dwarf water lettuce. They are surviving but not really going all that well. I think partially because there is to much flow at the top, partially because there isn't enough light.