You should have some target amount of salt ppm per gallon of water. This amount will change depending on A: the type of salt, and B: what you actually will have in your tank. API aquarium salt claims it's 1/2 tea spoon per gallon. Follow the guide on whatever salt you have, and for whatever animals you have in your tank, but let's use API as a base line.
In this case, you have, what is theoretically perfectly balanced salinity from tank A (5.5 gallon). Then you have empty 8 gallon tank B, but let's assume for generality sake, tank A has arbitrary volume that is always smaller than B. Let's call these values volumeA, and volumeB, where volumeA < volumeB.
You will need some method to remove the water.
- Gather supplies to remove the water, plus nets or other equipment to move the fish later, leave water for 24 hours before hand for use to top off the tank as well (24 hours to get rid of chlorine, conditioner has side effects in this case for a new tank). Milk cartons will work for holding the soon to be de-chlorinated water (make sure to wash out first).
- If tank A is at a higher level than B, you may be able to simply use airline tubing later on and let gravity slowly take water from tank A to B.
- If not, you can use the same method to drain it out into a separate location, like a bucket, but make sure you either have a big enough resevoir (i.e. like a 10 gallon bucket) or can stop the flow before all the water comes out.
- You can use a gravel siphon to accomplish the same thing at a much higher flow rate.
- You may need to put your mouth on one end of the tubing to get the water to come out the other end if whatever water transport tubing you have does not have a pumping method.
- Next set up tank B with whatever hardscape you are about to put in it.
- Consider what you are going to do with your animals.
- If your animals can withstand decent amounts of time out of water, remove them and put them in a safe location.
- If they cannot, either move them to a safe submersed location, like another aquarium, or if you do not have adequate space or equipment to move them temporarily don't drain the water all the way on the next step (half way for example).
- Take your bucket and drain tank A by sticking one bit of airline tubing/ siphon in your tank, and placing the other end at a lower elevation into the bucket. You need to get water from the tank to get "over the hump" at the top of your tank in the tube to force gravity to suck the water out automatically. You may have to physically suck on the tube on the other end, so you may want to clean that part. Remember not to take out all the water while there are still animals in the tank.
- If your bucket is not big enough to hold all the water, observe the tube, put your thumb on the end when the bucket is full enough, and place the tube back at the higher elevation.
- You can also use a cup to perform the same task, it just takes way longer.
- Take your bucket and do one of the following for your new tank B:
- If you just dump the bucket in, even carefully, you will likely ruin your hardscape, plants, corals, etc.
- If you are able to place the bucket at a secure higher location than your tank B, you may be able to use the same siphon technique to simply automatically have the water pour out. If you use a small standard airline tubing (3/16") then the out-pour of water should be weak enough to not ruin your tank design. If you only have access to a gravel siphon or similarly wide tubing apparatus, then the flow will be too strong. Consider combining siphon output with the next option in order to lessen the flow.
- Alternatively, take your bucket and pour it into a hand held water sprinkler. If you do not own one, you can create one out of plastic cups and either drilling holes evenly spaced at the bottom or taking a box cutter and carefully puncturing and turning it back and forth underneath the cup. This will weaken the water output and cause the blunt of the force of the water pouring to hit the can/cup instead of your hardscape. The same method can be used for more gentle water changes later on as well.
- If you do not have access to any of this, use a small 20 ounce cup or smaller to perform the same task (the size doesn't really matter, but you will have a hard time getting all the water out with larger cups and harder to control the flow).
- After draining all the water out from the bucket, see if enough water is in the tank for your pets (half is generally good enough for any tank size, but use common sense, wait if your fish actually won't fit or you think they could be harmed by you filling the water up), and if they were not put into separate temporary holding take them out of your other tank and put them into this one.
- Continue this process of draining and filling until most of the water is out of the other tank.
- If the water level is low enough that you need additional salt water, reserve some water for use for filling. If you use typical water conditioner now, you will release ammonia into the water and potentially kill everything in your tank (as a byproduct of the chloramine/conditioner reaction). Consider leaving water out for 24 hours beforehand in order to provide ammonia free water to add to your tank. For established tanks, this is usually less of an issue, because bacteria will consume ammonia in the nitrogen cycle.
- Mix de-chlorinated water together with salt.
- Using the same process discussed earlier with draining water into the new tank, take this new salt water and drain it into tank B.
- Continue this process until the water level is adequate for your needs.
- Put pets into new aquarium if they have not already been placed inside.