A thermometer that just gives you a number is only half as useful as one that also tells you whether that number is a problem — most owners do not have a tropical species' safe range memorised, which is where a marked zone on the thermometer itself does the interpreting for you.

Marina's Floating Thermometer uses suction cups to stay in position in the tank, and gives readings in both Celsius and Fahrenheit. It includes a marked safety-zone temperature range specifically for most tropical fish, and Marina list it as suitable for both freshwater and saltwater aquariums.

• Floating design, held in place with suction cups
• Reads in both Celsius and Fahrenheit
• Marked safety-zone range for tropical fish
• Suitable for freshwater or saltwater aquariums

What's not stated. Marina do not give the exact temperature values marked on the safety zone, so check the thermometer itself once it arrives rather than assuming it matches every tropical species — some fish need a narrower range than the general zone shown.

Who it's for: tropical fish keepers who want an at-a-glance way to check the tank is in a safe range, without converting a Celsius reading against a chart every time.