I have tried Aquapharm, Seachem, Tropic Marine, Salifert & Koralin and here's my 2 cents.
Aquapharm - Cheap but seems diluted, not much info
Seachem - Reasonably priced, name of products too confusing, state effective ingredients eg. Reef Calcium, Reef Complete, Reef Advantage Calcium -
Salifert - Resonably priced, seems diluted, state effective ingredients
Tropic Marine - Concentrated powder form, state effective ingredients, slightly pricey, powder cake easily
Koralin - Concentrated powder form, some packaging only have German text, pricey, cake easily but not as bad as Tropic Marine
Conclusion: Prefer Koralin due to its potency and results.