Male guppies dying

I have bought close to ten guppies over the past 3 months and they have all died. They usually appear healthy for a couple of days then their colour goes darker and they hang around the top of the tank, then start to swim funny then eventually sink to the bottom, occasionally moving but then settling on the bottom again all the time and die. The sicker they get, the darker their colours get and fins start to rot off. A couple of times i have taken a sick fish out and put in a separate more salted small tank with a tonic and melafix but they have all died anyway. All guppies so far have been male which all died. A week ago i decided to get one male and a female and now the male is sick (in separate small tank close to death) and the female seems fine. I also have 2 cory julies in the tank which are fine and that is all -so only the 3 fish at the moment :(. The ph is always around 7.5 and nitrates are always low. I don't know what i am doing wrong and the pet shop which i buy them from don't lose hardly any- as i will see the same healthy fish week to week and then take it home where it dies! I have an airstone, heater and good filter so i don't know what the problem is with my tank. I also float them well before putting them in my aquarium. I hope you can help because i really don't know why they are dying and what i can do to remedy the situation.
Thanks to anyone who can advise me

Its parasite. Use formilim at rate of one drop per gallon.

You can also try using garlic juice in the water. Garlic is good for parasites if that is what it is. You can also treat the tank with it occasionally just as a precaution.

Thank you both. If it were a parasite would i be able to see anything on his body or gills? At the moment i can't- only that his colour is a lot darker than previously. After i put him in the hospital tank and treated with api melafix he seemed to pick up but today he has gone downhill again. I've just grabbed some api pimafix and anticrustacean parisite and added these to the water too. Ive have also added them all to my main tank as well to hopefully eradicate anything going on there. How do i make garlic juice? Do i just crush a garlic and mix it with water or can it be bought specially for fish?

I was just wondering if my cory juli was the culprit? Do catfish sometimes harbour diseases as i didn't lose any guppies before i put him in the tank (but that was only a week after i initially got my guppies). If so, should i get rid of the cory juli or will it all be alright after my tank is treated for this mystery disease?

You can buy crushed garlic at the store in a jar. Pour some of the juice from that and add a about a tsp. of it to the tank.

Well he's dead. Maybe i put a bit too much med in his hospital tank i don't know. My female guppy and my cory juli is fine and I'm treating the whole tank as it says to treat for 7 days. I just want to make my tank safe for when i introduce new guppies in say a months time when everything settles down. I am also thinking that i may take the cory juli back to the aquarium as it may be harbouring the parasite or whatever disease the guppies are dying of- do you think this is a good idea?

Also- I'm not sure if their fins actually rot off from the illness or are damaged from the other guppy/ies bumping and pestering them to move when they are sick.

The parasite attacks the tails of male guppies. This is known by importers and wholesalers. Treat with formilin for 3 days. Parasite only attacks male guppies. Rest of your fish will be ok. If you buy more males they will succome to the same fate. I have never tried garlic or any of the melix ptoducts.

Thank you. I am treating my main tank at the moment. I only have my pregnant female guppy now.

I don't have formalin at my local aquarium but i think the ones i am using are equivalent. Melafix is for bacterial infections (active ingredient is melaleuca 1%) and Pimafix is an antifungal (active ingredient is pimenta racemosa 1%). I also have Anti-crustacean parasite (active ingredient is sodium chlorite 12,835mg) which i am using but unsure because it says only to use if the fish have visible parasites (which they were not).

If the treatment does not work, I was just wondering if you know how long it would take for the parasite to die off without any male guppys to attack? Would they still survive by infesting the other fish (my female guppy) who may not die off because of being more resistant?

Sorry for being so analytical but i just want to get this right to that i can keep my female healthy and get some more male guppies in the future.

Neither of your two meds will work. Formalin is what you will need. Will the parasite die off. NO. The parasite attacks the tail of males and turns them black. Then male dies.

I've also been putting in a tonic (interpret general tonic) which says it is also for parasites- it contains formaldahyde so should be similar to formilim.

What i meant by the parasite dying off is that if it had no male guppies to feast on would it die off? -Because it doesn't have any food.

If the meds do what they say they do then I shouldn't have to worry because the 3 meds combined should kill everything. I'm treating the tank for a full 7 days and following instructions to the letter.

Thanks for all your help. I will let you know how it goes.

I use the term formilin because thats what most hobbist would know. When I treat I really use formaldahyde straight. One drop per gallon. All I know is that if you re enter males to that tank they will come down with it again. When I have that or any other problem I sterlize the tank. Once didn't and put other fish in the tank. Everything was ok. When they were gone I got another shipment and put them into that tank and they came down with the same thing.