Stop buying medications and don't put anything else in the tank
What is metro? If it's metronidazole then that should only be used as a last resort if the fish have internal protozoan infections, unlikely in this case.
If the fish are still eating well and doing stringy white poop, then it sounds like they have intestinal worms, either thread/ round worms (capillaria/ camallanus) or tapeworms. They can have both thread worms and tapeworms at the same time. You need different medications for each type.
Praziquantel is for tapeworm, Levamisole is for thread worms. There are plenty of medications but you will have to check to see what is available near you. There are other medications with different ingredients than Praziquantel and Levamisole and they also work. Flubenol was one that was recommended 10yrs back and is meant to work.
Most deworming medications need to be used weekly for 3 weeks. This is to kill any adult worms and any eggs that hatch after the adults have been dealt with.
Tapeworms can be treated with Praziquantel.
Use 100mg of Praziquantel per 20litres of water. Do a water change 24-48hours later. Then repeat the treatment a week later.
You can also crush Praziquantel up into a powder and mix it into some frozen (but defrosted) fish food. Then feed the fish as much as they can eat. Use 1 tablet of Praziquantel for 1-2 heaped tablespoons of frozen food. Repeat after a week.
Levamisole hydrochloride is available in 3 concentrations. The lowest concentration is for birds/ poultry, the middle strength is for sheep and the strongest concentration is for cattle. You use different dose rates depending on which strength you start out with. The dose rate for bird/ poultry is the one you will need to find because that is the Levamisole you currently have.
Levamisole Hydrochloride is fine with all fish including catfish, loaches and other scaleless fish.
You must do a partial (50% or more) water change 24 hours later otherwise small fish like neon tetras start to have problems and become nervous and skittish. If you still don't do a water change then the small fish might start to die after a few days. But as long as you do a partial water change 24 hours later they will all be fine.
Levamisole is available in different concentrations. Depending on which concentration you have will determine how much you use.
Pretty sure the one I used was for sheep and contains 32g/l of Levamisole, and I use 5ml of that per 10litres of tank water.
I think the birds/poultry medication has 16mg/l of Levamisole. If yes, then you will need to use 10ml of the Levamisole per 10litres of tank water.
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re: The acara with its protruding anus. Are there any thin white, red or grey filaments hanging out its butt? If yes, then these are thread worms. If no, then the fish might simply be getting ready to breed. They have an ovipositor (thin tube used to lay eggs or sperm) that pops out shortly before they breed. It will retract after they have bred. If it has been out for a week then it is not breeding and most likely internal problems like worms.
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To work out the volume of water in the aquarium:
measure length x width x height in cm.
divide by 1000.
= volume in litres.
When you measure the height of the tank, measure from the top of the substrate to the top of the water level.
Remove any carbon in the tank/ filter before treating and throw the old carbon away. carbon will absorb medication including dewormer.
Do a 75% water change and complete gravel clean before treating. And 24-48 hours after treating do another 75% water change and complete gravel clean.
Make sure any new water going into the tank is free of chlorine/ chloramine before it is added to the aquarium.
Keep the feeding down to a minimum when treating the fish.
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lots of edits, sorry. I was going thru my old posts on the forum to find the info about dose rates.