It is rather senseless to consider frozen fish as being lower quality. It is going to take a good few hours for saltwater fish to make it to our dining table so freezing is the surest way to preserve its flavour and the health-beneficial nutrients it contains. It does, however, depend very much upon the method by which it is defrosted. The fundamental rule here is not to rush.
We ought to include marine fish in our diets as often as possible. From a nutritional standpoint they are highly valued and the consummation of fish is beneficial for our health and so they belong not only to our Christmas menu. The problem is that fish flesh succumbs to deteoriation relatively quickly. The reliable way to convey fish from distant seas to the Czech Republic is by its freezing. Ideally this should take place as soon as possible after it has been caught. For this reason a good choice is fish marked by the abbreviation FAS ("Frozen at Sea") or "Frozen at harbour" - both methods are explained in detail here.
Importing chilled saltwater fish to the Czech Republic is much more challenging. The chilled fish which are offered by certain retailers are frequently just defrosted and placed on ice. For a considerable surcharge, naturally. And so it is senseless to refuse to buy frozen; rather the opposite. The key then in terms of the flavour and nutritional value of the resulting dish is the correct procedure during the defrosting and the preparation of the fish.
The flesh of fish is one of the most tender and and its structure is very easy to disrupt. For this reason it is necessary to proceed cautiously with its defrosting and allow the process sufficient time. The ideal way to defrost fish is to leave it for a number of hours in a fridge. Put the fish on a plate and leave it in a fridge overnight to defrost at a low temperature. Even quality fish will release water. It applies nonetheless that the more water it releases, the less quality is the fish, its grammage supplemented by excess liquid. Where possible then, place the fish on a ribbed surface so the flesh will not be sitting in water. Once readied in this manner, prepare the ingredient quickly, within 24 hours of its having been defrosted.
If you want to speed up the defrosting of the fish a little, go for ice water. It's sufficient to immerse the frozen fish into this. Ideally then, refresh the water every half and hour; the fish ought to soften within tens of minutes.
Substituting hot water for cold is, however, certainly not recommended. Defrosting would, it is true, be sped up by this, but this is definitely not recommended. It is one of the reliable ways to damage the flavour. The thing is that fish flesh is very quick to prepare and the hot water acts on the fish by partially [cooking it]. Such an impact on the structure of the flesh is not something such a delicate ingredient can typically recover from. Neither is a microwave the best of solutions. Though it can be of help in the defrosting of robust meats, it makes a muddy nothingness of fish. Better to give the microwave a break in this instance. Fish simply needs time and efforts to speed it up will not lead to success.
Curiously, it is possible to speed up the process, though only for smaller, or rather thinner pieces of fish. These ought to be no more than 1 cm thick. Then you can put them straight from the freezer to the pan. These thin pieces will heat through quickly enough to make them evenly cooked. Do not, whatever you do, try any such thing with a tuna steak: the result will be the burned or completely dried out surface and frozen centre. And that would be a real pity. The preparation of thicker portions of fish demands flash frying at a high temperature. You can certainly try this method and experiment; you will find inspiration and recipes for the "Cook It Frozen" technique which are tailored to the cooking of frozen fish.