Firstly welcome to the board Leonclaro,
The interest we express in your problem is the same we express in everyones problems, we are here to help and assist in any way we can. OK now on to your problem:
1. Water Hardness:
What is Water Hardness?
Water 'hardness' is predominantly caused by the existence of the minerals calcium and magnesium in ground and surface water. If either or both minerals are present in your drinking water in high concentrations, the water is considered 'hard.' These minerals come from sedimentary rock such as limestone that dissolves in our river system, causing an increase in water hardness. The result of hard water is difficulty making lather or suds for washing and a build-up of minerals on taps and on other fixtures.
Water containing low concentrations of calcium or magnesium is called 'soft' water. Municipalities with soft drinking water often have higher incidences of water pipe corrosion (low pH).
In order to test the hardness you would need to get a kH
test kit, I think I have never tested my hardness, and you would need to try and match the hardness to your type of fish.
2. Your water change cycle.
I would not remove any of the decorations or the plants or anything when doing a water change as these things hold or contain the important bacteria that 'eats' up all your ammonia and nitrites that are created from your fish. The way you are cleaning your tank at the moment, your tank has to recycle everytime you change the water and this could be the cause of your fish deaths.
The way I would recommend that you use to change your water would be as follows, bare in mind that this is not set in stone and other people may have different ways, I use this method and it works for me.
The day before or at least 12 hours before you change your water, 5 gallons, get a 5 gallon bucket or container fill it with water, cold water only, add your dechlorinator and any medicnes, pH buffers and things to control your water chemistry, that you might be adding if need be. Put in a heater and let the water stand for, like I mentioned at least 12 hours, this will give the water enough time to warm up and any 'bad stuff' to get removed by the dechlor and medicines.
Second - When cleaning your tank do not clean the filter and the gravel at the same cleaning, or lightly 'vacuum' the gravel. When it comes to cleaning the filter I normally wait a couple of days after changing the water to change and clean my filter, but I also have two filters on my tank, a 130G tank, so I would clean the filters on alternate weeks. I only clean some of my ornaments when I rearrange my tank and then I time the ornament clean with a water change so I can fill a bucket and put all my ornaments in there to keep the bacteria alive. Just before I put them back in I just give them a quick shake in the water them put them back.
REMEMBER to turn off your heater when doing the water change, depending on how you have your heater in the tank, I have mine lying horzontal to the gravel on the bottom of the tank so I don't have to turn mine off.
Hopefully this will assist you in cleaning your tank and keep your fish alive. BUT remember this is how I clean my tank and not necessarily how everyone else cleans theirs.
Another thing your bala sharks are going to out grow your 20G in no time at all, they grow to 14", I would look at getting some smaller fish.
See this link on the shark:
Hope this helps, but wait for more in fo
from some of the other guys as well.