What sort of fish do/ did you have?
How long did you have the fish for?
What symptoms did they have before they died or when they were dead?
Do you have any pictures of the sick, dead or live fish so we can check them for disease?
What are the tank dimensions (length x width x height)?
Did you gravel clean the substrate when you did water changes?
How often and how do you clean the filter?
Do you dechlorinate the tap water before adding it to the aquarium?
Do you have chlorine or chloramine in the tap water?
Do you treat the new water for chlorine or for chloramine (they normally require different amounts of dechlorinater)?
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It's a good idea to do partial water changes on a regular basis to dilute nutrients & disease organisms, and keep the water clean. If you just top up the evaporated water, you leave all the bad stuff in the tank and that can kill fish.
You need to do water changes regularly even if the water tests are good. There are lots of things in water that we can't test for and they need to be removed as well.
You do water changes for a number of reasons.
1) to reduce nutrients like ammonia, nitrite & nitrate.
2) to dilute disease organisms in the water.
3) to keep the pH, KH and GH stable.
4) to dilute nitric acid produced by fish food and waste breaking down.
5) to dilute stress chemicals (pheromones/ allomones) released by the fish.
6) to dilute un-used plant fertiliser so you don't overdose the fish when you add more.
7) to remove fish waste and other rotting organic matter.
Fish live in a soup of microscopic organisms including bacteria, fungus, viruses, protozoans, worms, flukes and various other things that make your skin crawl. Doing a big water change and gravel cleaning the substrate on a regular basis will dilute these organisms and reduce their numbers in the water, thus making it a safer and healthier environment for the fish.
Imagine living in your house with no windows, doors, toilet, bathroom or anything. You eat and poop in the environment and have no clean air. Eventually you end up living in your own filth, which would probably be made worse by you throwing up due to the smell. You would get sick very quickly and probably die unless someone came to clean up regularly and open the place up to let in fresh air.
Fish live in their own waste. Their tank and filter is full of fish poop. The water they breath is filtered through fish poop. Cleaning filters, gravel and doing big regular water changes, removes a lot of this poop and harmful micro-organisms, and makes the environment cleaner and healthier for the fish.
If you do a 25% water change each week you leave behind 75% of the bad stuff in the water.
If you do a 50% water change each week you leave behind 50% of the bad stuff in the water.
If you do a 75% water change each week you leave behind 25% of the bad stuff in the water.
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The best thing to do for a tank that has old tank syndrome (caused by lack of water changes and simply topping up evaporated water) is small partial water changes. Do a 10% water change and gravel clean some of the substrate every day for a week. Then increase it to 20% every day for a week. Then 30% every day for a week. Then do 50-75% once a week after that.
The small water changes at the start help to dilute things slowly and won't put the fish into shock from a sudden change in water chemistry (pH, GH & KH). Doing small changes regularly over a period of weeks slowly makes the tank cleaner and once it's clean, then you can do bigger water changes.
Make sure any new water is free of chlorine or chloramine before it's added to the aquarium. If you're unsure of whether you have chlorine or chloramine in the water, contact your water company by phone or email, or check their website to see what they add to the water. Then treat the new water for that.
After you have cleaned the tank up and things have settled down, then look at adding new fish if you want more.
Fish can tolerate huge water changes if the new water has the same chemistry as the old water. If the tank has been left for a while (like yours has), then you have to get the tank water the same as the tap water and you do that slowly. Once they are the same, you can do a 90% water change every day and the fish will be fine. You don't normally have to do water changes every day but fish can take it easily as long as the new water has the same chemistry and similar temperature to the tank water, and is free of chlorine/ chloramine.
