Forum members here will post candidly about what is right for the fish, we are a passionate group and sadly fish cannot speak to us to tell us when things are not right.
For what it's worth, I urge you do the following ASAP before you goto bed tonight...
- Unplug the heater (if there is one) and give it at least 20 minutes to cool down.
- Then unplug the filter, as it will get noisy and struggle as you lower the water level.
- Remove almost all the water, literally leaving just enough to cover the fish. They might stress in the short term, but they will thank you with their lives shortly.
- Add similar temperature, dechlorinated water back into the tank. Unless your filter is caked in rotting food, your fish should now have water that has almost nil ammonia.
25% water changes by definition will only lower deadly toxins like ammonia and nitrate by 25% at best (pH and temperature do play a part in this, but it is a reasonable ballpark). Until you invest in a liquid test kit, you need to be doing at least 50% water changes daily, test strips are infamous for dodgy results.
Have a look on Ebay for a size of Seachem Prime that suits your budget. This dechlorinator is meaga-concentrated and cheap compared to other common ones, 5ml is needed per 200l of tap water. In addition, it can be overdosed to help "lock" ammonia and nitrite into less dangerous forms for 24 hours.
Pop along to a local fish store or again, look on Ebay, to buy 5-10 bunches of Canadian Pondweed. With a lighting period of 6-8 hours, these floating plants will use ammonia and nitrates as food, while also oxygenating the water. For even better results, buy a bottle of Easycarbo and Prolito, as these will ensure the plants do not run out of essential nutrients.
In conclusion, cycling a filter with fish in the tank is much harder work and will take much longer to cycle than by the "fishless" method, providing you have the interests of your pets at heart.