Fish keep on dying when I clean tank?

Sounds like overcleaning.

Use the gravel vacuum to remove 25% of the water once a week while sifting through the gravel. You dont need to clean the dechorations, dont change filter cartridgees, dont clean the filter,

Get a digital thermometer, make sure that the new water, is the same temp as the rest of the tank water! Add new water slowly and make sure that the tempurature is staying withing 1-2 degrees. of its normal temp.

Are you adding othe chemicals? like ph changers? NEVER use ph changing chemicals.

Add seachems prime to all new water. put it in the tank before you put water in the tank.

cleaning out the tank can be stressful to fish. Leave the decorations and hiding things in the tank for them while your changeing the water, if they need cleaning - they raraly do - do it on a day you are not cleaning the tank, and dont do all of them at once.

Only change filter cartridges when the are falling apart, they can last 6 months to a year. But you will want to rinse the it a little bit, in a bucket of used tank water, when its very dirty. when you do need a new filter cartridge, cut some of the spong of of the side of the old cartridge, and stuff it in the filter, dont rinse it, dont let it dry out.

One more thought -

Did you add seashells to the tank? or crushed coral, or somthing? these things effect yoy Ph, sudden changes in ph can shock the fish and kill them.

I suggest you test your tank ph before a cleaning, and you tap water ph, "if thats the water you use in the tank"

Okay. First off, NEVER take out more than 1/3 of the water in a tank-more will unbalance the tank and it may need to 'Re-Cycle'(and put your cleaner into the gravel to get that icky black stuff out). Don't know what Big means, but I run a 75 gallon myself. You can scrub the decorations with a toothbrush and put them back in. Rinse the filters-Do Not replace them till they are falling apart.

Next, go to Home Depot and buy Straight Razor Blades(if you are underage, let Dad buy them for you). Scrape the Inside of the tank-more crap ends up on the glass than you might think. Let the filters suck it all out.

1. Not cleaning enough.

2. Changing too much of the water.

You need to clean the tank every week.

Only then is taking 1/2 or more of the water even close to okay.

And don't bother changing the pH. That kills your fish.

Overcleaning is more likely to kill fish than never cleaning at all.

You are maintaining the tank wrong.

are you re-acclimatising the fish to the temperature of the "new" water?

Get a large bucket, fill it with your new water and let it come to room temperature (could take a couple of hours)

before adding it to freshly-cleaned tank.

Maybe the same reason the crickets I feed my bearded dragon keep suiciding into the water. They just want to end it all.