do i need to pay someone to clean it and take care of it, im asking because one of my friends has a 123 gallon fish tank(but no oscars) and everytime they tried to clean it themselves the fish died so now the just pay some service

no..... if you follow the simple procedure listed in this site,
you will not get wrong about it.... i am not sure how do your
friend clean his tank... but that is the weirdest story i've
heard in fish keeping....

Hello and welcome to you..

No you can absolutely do it yourself! I would put money on no de-chlorinator!

or he washed his filter and media ........ DOH !!!!!!!!!

There are few very simple rules you must follow. If you do things wrong your fish will die as they need water safe for them to live in. This dose not mean tap water is safe for fish even though you can drink it. you 1st need to add treatment (de-chlorinater) to tap water before you put it in your tank and only change one third of the water each time. if you mess up and get tap water in your filter your filter will not work as it should as untreated tap water will kill the good bactiria that live in the filter, this will lead to fish death too.

step 1 remove 1 third of your tank water. or less if you like.
step 2 turn your filter off. BUT ONLY FOR A SHORT TIME 15-30 MINS. will be fine
step 3 add treatment to your tank to remove chlorine and make tap water safe.
step 4 fill the tank up to level.
step 5 TURN FILTER ON.

it sounds easy but 1 third is a lot of water to remove by bucket so a hose to syphon it off will be needed. also a way to put the new water back i would start small and remove a little more as you get use to it.

WE ARE TALKING FREASH WATER FISH OR TROPICAL?? AND NOT SALT WATER HERE.
I would recommend they watch the guys that do the service a few times why not your paying them.)))) At some point you will need to clean the filter but we will need to know what filter your friend has. This needs a little more care but again is easy to do by most people if done right. please ask before you attempt it. if you are not sure ask for more advice

My money says he removed all the water of tank

i have to agree with nickie and BGH sounds like your freind didn't add dechlorinator and he washed his filter and media in tap water most folks here have much larger tanks than 125 and they do it themselves as marcus has stated it's all quit simple to clean then tank when done properly