So here's my long story... and I appreciate that your taking the time to read this.
I've kept fish all my life, I'm no newbie. Mostly tropical freshwater fish. I know the nitrogen cycle, temperature, parameters, and everything necessary for fish.
I need your opinion.
1). Is changing my 100 gallon fish tank water once a week too much?
2). And does the city add more chlorine to sink water during the weekends than weekdays? How can I test that?

I change about 15-20 gallons on any WEEKDAY once a week with tap water and NEVER used my dechlorinator. I used Prime brand, novaqua, or API stress coat with Aloe. I'd thought chlorine would evaporate after a day or 2 and I NEVER had problems. I only used the chemicals if I thought of it. Fish were always happy and thriving. Everything was always going great....
Until I added table salt which contained Ioidized substance (I also overdose putting atleast 8 or 9 cups of 5ml). I noticed one of my fish was breathing hard after day 2.
I researched it and yes... really big mistake.
I moved all of them into a 10 gallon where as the water was rarely added with chemicals. they were doing fine in that tank.
I removed about 40% of the water in the 100 gallon and replaced it with tap water during the WEEKEND. I conditioned it with Prime and API stress coat and dechlorinator and it was running for 2 days.
I thought I'd used that water and change 20% of the 10 gallon water because it was already conditioned. (the 10 gallon was overstocked so I knew I had to replace some water so ammonia or nitrate levels don't get high)
4 of my fish (3 clown loach and datnoid quickly went into shock!! :(
I had another Datnoid and 2 parrot fish left which were bigger in size. I remembered I left my old water in a cooler because It was too heavy to lift when I wanted to empty it. I used that as a storage. I didn't trust it at first because it was really old water but it was all I had.... and I put them in there. added filter, heater, and a powerhead that created bubbles at the surface and high flow of water to create some sort of oxygen and cycle. It saved them... what a relief... Fish owners, this is a must if you throw away the old water or no other source!!! pets question and answers,www.5d2d.com
those other fish went into shock so fast within couple mintues. I couldn't react fast enough because I was panicing. I'm so bummed right now!!
Well that was my story. I'm scared to buy more fish because I always do something wrong sometimes. those fish were pretty pricy!! I know better now of table salt.... I added salt in the first place to add slime to my fishes so they'd be happy. I think those fish was better off left alone and die slowly of high nitrates haha.
5 stars to someone who makes sense.... and I will appreciate any answers and advice. I will always consider changing my methods of keeping fish.

to be perfectly honest, you learn by your mistakes, you wont make it again, and if something similar goes wrong you know how to deal with it. also unless you're using a pretty rubbish filter id say once a week water changes is unnecessary i take about 20% if that out of my three hundred liter tank once a month and clean the filter at the same time and its running fine, always has bearing in mind it is fully stocked. pets question and answers,www.5d2d.com

with regards to chlorine, the best way is to test it yourself, buy some test strips and test each day for a fortnight just to make sure, recording the readings and compare to see when, personally id use a dechlorinator but its down to you.

bear in mind that if there is a water shortage or problem they may well get water from elsewhere, this will have a different chemistry which could cause problems