Ok so I have a silly question you may all say but I have a 75 gallon that is working fine now but I got a 300 gallon tank too but not in use since only place I can put it is where my 75 gallon tank is at. I know that to reduce the stress on moving fish is to have the water saved from one tank and move to another but since size difference is so drastic and saving 75 gallons of water for the move is what I would really like to do but is not an option with buckets and such and moving one with any water what so ever is a killer on backs and such. I was wondering on how I should go about it and the only thing i can think of is to take my bed apart and place the 300 next to it and then fill my 300 with the water and all that but I really dont want to do a double move on the fish I want to move one time and quickly and cycleing the tank is needed too since it is new? any ideas or suggestions would be greatly accepted

i have two tiger oscars a shovel nose catfish and pike chichlid, and pleco, all no bigger than six inches

Unfortunately, swapping tanks, especially large tanks is never an easy operation. I've done it a few times and it's a job that needs to be done properly so don't take any short cuts. I think your biggest problem is saving the bacteria in your existing filtration system. To help with the cycling on the Newtown, you really need all those bacteria alive and kicking. Is there any way you can get a very large reciprocal that you could fill with existing tank water and keep the filtration running on it until the big tank is full and ready for the filter? a lot of people keep what we call water butts here in the UK. Basically they are large barrels, normally made out of plastic that are used to collect rainwater. One of these would hold enough water for you to run your filter on until the big tank is completely full.

Because you want to put the big tank where the smaller tank is, you are going to have to dismantle the whole lot so I would definitely think about using something to keep your filtration running. I wouldn't worry about anything else, keep as much water from the 75 gallon as possible. However, it's not going to be a disaster if you can't, just make sure you use a water condition now before starting up the filter again on the new tank. In fact, I would probably advise you to put some stress zyme in. Failing that, prime or stability will do the job just as well

Siphon some water from the 75 enough to place your filter media in.

Move fish to a bucket with tank water.

Empty the 75 and move it from the space it's in.

Setup the 300 add gravel and anything else from the 75 such as decorations or heaters etc... Fill it (set the temp of the new water to same as old water) and dechlorinate the water.

Add filtration and any water you have from the 75 and you should be good to go. Add the fish.

The whole process is just like doing a 100% water change on an established tank.

There is very little if any good bacteria in the water itself most of it is in the filter media and gravel.

If you cant save most of the water dont worry. As said most of the good bacteria is in the filter. Take everything except the water and the filter out first. Then take as much water out you can with the filter running. Then remove the filter and the rest of the water. Swap the tanks over and refit the new tank in the opposite order to dismantling the old one. You want the filter to be off for the minimum time but you have a couple of hours maybe more