I thought I'd share a little experience I went through recently.
Back last Winter my long established tank began clouding up for days after each weekly cleaning et partial water change. When I tested the newly changed water it tested high in ammonia then I found the tap water itself was testing high right out of the faucet. I'd been using API's Tap Water Conditioner for years with no troubles (or so I thought) but throughout the couple of months prior to this weekly cloudiness with ammonia spike my oscar swiftly developed bad HITH and died.
After some research I found my local water municipality had begun using chloramines rather than chlorine to offset all the nasty run offs we've apparently had here into our water supply and API's Tap Water Conditioner doesn't break the chloramine bond nor does it neutralize ammonia resulting from that.
Other fish keepers I met online turned me on to SeaChem's PRIME which not only breaks the Choramine bond but converts the resulting ammonia into ammonium which is much safer for fish and can still be consumed by the biological filter. I switched to this immediately and I'm sure saved the life of my new oscar though there was still an issue with weekly clouding because of all the fresh ammonia introduced into my tank with every partial water change.
So I then got a 30 gallon plastic tub, placing it on my metal stand's lower level I filled it with water which I pretreated with Prime then ran a Fluval submersible filter in full of zeolite to get rid of the free ammonia before it went into the tank. I refill the holding tank after each partial water change and it runs through the zeolite filter for a week. I even have a heater in the holding tank so the water will be the same temp as the tank water when I refill it.
Some have told me I'm going over the top and don't need all this fuss but I can't help but think weekly bacterial blooms from large doses of tap water ammonia (or even ammonium per the PRIME) can't be good.
I just want to raise my new oscar in as perfect conditions as are possible and I certainly don't want to ever see HITH again.