The API kit is worth investing in, trust me! The dip strips often appeal to people at first (I bought those when I first set up a tank too!) Because they're cheaper, and they look easier. Not many of us got into the hobby to play with chemistry! But you actually get many more tests out of the API kit, so it really is better value for money in the long run. Crucially though, it's much more accurate, which is important when things are going wrong and you really need to know the right numbers.
Salt water treatment can really work miracles with issues like this. If you follow the plan outlined by Colin above, it'll give your guppy the best chances of recovery, and reduce the chances of any of the others falling victim to the same problem. Clean, fresh water is the best medicine. I cured this guppy who fell out of the tank and tore most of her tail off, and developed a fungal infection all down one side of her body, just through water changes and salt treatment alone.
Day after she fell from the tank. She was somehow caught up in the sponge filter when I lifted it out for cleaning, fell to the tank stand below and was only out of the water for seconds, but by the next morning, a fungal infection had taken hold at the remains of her tail, and along the side of the body you can't see in this pic. I began water changes and salt water treatment.
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Within days, the fungus started to reduce, and her tail began to grow back. A year later, she was my largest, healthiest female, with only some scar tissue on her tail to show for her ordeal
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