I have 3-4 kg size fish in my non-frost-free deep freezer. They always stick very hard to the base of the freezer and it takes me hours to get the fish out. I have to keep the freezer turned off, which affects other fish or food items.
Is there a way that I can get the fish out in a much shorter amount of time, so that I can thaw it outside of the freezer?
For the already-stuck fish: no, there is no way. Don't try to chisel them out, I've known somebody who damaged their freezer that way.
For any fish you will be placing there in the future: find a packaging material which will not stick to the freezer. Plastic bags are the most common way of doing it.
You also have to ensure that your freezer's bottom is dry. It is OK if it gathers a bit of loose frost over time, but there shouldn't be a big lake of frozen liquid on the bottom. If that has happened already (for example because you held in items which leaked, or because you defrosted it and didn't dry it before it froze again), you have to defrost the fridge until the complete lake is melted, then remove all the liquid, then let it freeze empty, without any food in it. Then package the food in liquid-proof packaging, and re-arrange it in the freezer.
You can sometimes break stuff free from ice, but you have to be careful not to damage the fridge, so you shouldn't use sharp tools.
What you want for something like this is a block of wood (like a short section of 2x4). Place the block of wood against the food to be removed, and then hit it with a hammer.
This won't always work. Some food will break before the ice, so you end up with broken shards of food. You can sometimes make use of this by using a more narrow board (like a 1x4), so you're trying to destroy the bottom layer of the frozen food, leaving the upper level intact ... but I wouldn't suggest using anything thinner (0.75", approx 1.9cm), as it will be more difficult to hit the board with the hammer ... and a miss could strike the freezer instead, and do a lot of damage.
You can also try to crack the ice to the left and right of whatever you're trying to remove, so you can try to remove everything as a huge chunk. We used to have to do this with an old freezer every six months or so as I was growing up.
I would also recommend looking for short wire shelves, so your food isn't sitting directly against the bottom of the fridge. This gives you some time before the frost has built up to where it's surrounding your food. And if you're in a humid area, try to keep the freezer full, so you don't exchange the space in the fridge with humid air each time you use it. Empty bottles will work, but bottles of water can also help in case of a power outage.
If you have a single layer of something that's frozen to the bottom of the freezer, you can put hot water in a container (e.g. a Zip-Lock bag) and place that on top of the item you want to unstick. This will apply a significant amount of heat that's reasonably localized to that one item/area, and isn't something which you have to be personally present throughout the time it's being applied. Depending on how much food needs to thaw in order to unstick the item, you may need to either replace or reheat the water (e.g. in a microwave), potentially multiple times.
This process will result in the item you're trying to remove being at least partially, if not completely, thawed, which should be accounted for in whatever plans you have for the item (e.g. the food, or at least portions of it, will likely be in the temperature danger zone by the time you're done).
Adding something hot to your freezer will increase the overall temperature in the freezer, at least to some extent. While the contained water should apply the heat-energy primarily to the item on which it is placed, there will be some which dissipates into the freezer in general. Care should be taken not to add too much such that the overall temperature in the freezer rises too high. Various insulating materials (e.g. a small insulated cooler placed upside-down over the items) can be used to limit this. You could even cut out the bottom of a cheap Styrofoam insulated cooler, which you then set around the item you're working on, so its used both for insulation and to help hold the container of water in place on the item you're working on getting unstuck.
As others have mentioned, it's substantially helpful to have the bottom of your freezer be dry, so be sure to clean up anything that's left melted at the bottom of your freezer at the end of this process. It may be beneficial to place towels around the item to soak up anything that becomes liquid while you're working on this.
I've even used this process to spot-defrost a freezer or just clean up water that's frozen in specific areas of a freezer without the need to completely defrost the entire freezer.
Answering my own question.
All the answers above are effective somewhat. But I have found full proof solution that will keep the fish/food from getting stuck. Its an easy process.
After placing fish/food in the freezer and couple of hours gone and the food are not stuck yet, try to move around the fish/food in the freezer. Repeat ithe process two/three times. And thats it. The food item won't get stuck anymore however long you keep them.
Besides the answers already given which address the long term solution, try using a hair dryer. Since it can be directed, you can target the items that you want.
Take everything out of the freezer and store it temporarily in boxes. You want to group small items together in bags, and then wrap everything (like every box or big bag) in towels or other clothing to keep it from defrosting. You will finally reach your fish at the bottom. Then use any combination of methods others have suggested, including chiseling out some of your catch if that is possible without destroying the freezer.
You may have to turn off the freezer and remove all the ice (and the fish). It should only take an hour or two with the freezer turned off to completely remove all the ice if you leave the lid open and add head with a hair dryer. Clean and dry the freezer and turn it back on.
The food you stored in boxes should be fine, but as for the fish, cook yourself a big fish dinner and invite all your friends!
If you can get a decent insulator (e.g. cooler), and the other items are significant thermal mass, you should be able to leave them outside the freezer for a few hours without too much trouble. If you want to give yourself a bit more time, you can set the freezer extra cold first, then take the items out. Adding water bottles and/or freezer pads first can also help, especially if the items are irregularly shaped. If you have a fridge, you can also stick them in there, as long as you don't have many items there already, and the ones you do have can stand being frozen; if you fill (emphasis on "fill") a fridge with frozen items, and don't open the fridge, the stuff should stay frozen for hours if not days, even without power.
Once you've gotten everything else out, if you don't care about your water bill, you can speed up the defrosting by pouring hot water and periodically pouring it out (or siphoning it out or otherwise removing it) and replacing it with new hot water. If getting water out is too difficult, you can put most of the water in bags and replace the water in the bags with hot water, and have just enough free flowing water to conduct the heat.
Another answer suggested a hair drier, but air has very little thermal mass, and so does little to defrost items. Simply getting a fan to circulate air will give most of the benefit of a hair drier (cold air sinks, so without circulation, even an open chest freezer will keep items frozen for a while).
For the future, you should wrap your fish in wax paper, and then place it in a plastic bag. You may also want to reconsider freezing 4kg blocks of fish. Smaller pieces will use up more paper and bags, but will be much less unwieldy and easier to defrost.
I think the easiest way is to pour hot water on it.
