Hi Mohamed,
None of these products are ideal for fish, however the best choice for Nitrogen, of the ones you listed is the Urea. The toxicity is much more likely to occur if the pH of your tank water is 7 or higher.
You also did not mention if your tank is CO2 injected or not. This will make all the difference.
If you are using CO2 then this will drive the pH below 7. You can dissolve 3 grams of Urea in 1 liter of water and dose 1 ml of this mixture per 50 L of tank water 3 times per week. This takes care of N, but P and K will need to come from a different source.
If you cannot find another source of P or K then you can use the 20-20-20 in very small amounts (I haven't done any calculations because the numbers do not tell you what salt comprises the numbers).
If you are not using CO2 then you can still use the Urea but you have to use even smaller amounts, like 0.3 grams to 1 L and you only dose this once a week or once every two weeks. Please note that the higher your pH the more toxic this will be.
I really don't like to recommend using these fertilizers because as other posters have mentioned, it's so easy to kill your fish if you make a mistake, and, you can also induce algal blooms if you are not careful. However, since you have no other option it might be worth a try.
Cheers,
