Can someone please help me sort out my 25 litre tank - it's overstocked, high ammonia, low PH, no bacteria?

First off. Is your tank cycled? The high ammonia may be another process of the cycle. If your tank was cycled. You may want to buy a protein skimmer. They are not harmful for freshwater tanks. All they do is rid of organic waste. That should take care of your ammonia. And your low PH. What kind of water are you using? Tap water? Maybe to much CO2. What are your parameters?