50 DS18b20 chips can be bought from about £35 on ebay (china), but you'd have to encapsulate and waterproof them, unless you can reliably read the outside glass temp. There are also encapsulated ones at 10 for £17. These will be the most accurate and easiest electrically to set up - its a one wire bus, though given 50 of them you will need to look at feeding them correctly with power (i.e use 2 wire mode, not parasitic power) and you may need to split them into groups. Beware of trying too high accuaracy levels as they take time to convert.
Next option would be analogue LM35s or similar, but as earlier mentioned you'd need lots of A/D converters, or some form of selecting circuit (see below) to select individual chips and send that signal to a single A/D converter. LM35s are about £21 for 50 on ebay. You'd probably need to keep LM35 chips powered all the time, so you'd also need something like 74HC4066 quad analogue switches to isolate individual LM35s.
Easiest and cheapest would be thermistors or thermocouples with a switch circuit to a simple A/D convertor. again you'd need to encapsulate them, or read the outside of the tank. You'll get 10 thermistors for a pound.
To create a simple swithing circuit you need either a set of counter/ripple gates (74HC595 £3 for 10) or some I2C controlled port expanders (PCF8574 £5 for 10). This would give you 50 pins. You write code that pulls just one of these pins high at a time, the rest have to be low. To each pin you connect a thermistor/thermocouple and then connect the other side of the thermocouples to the A/D convertor. This way 50 different sensors are cycled through in turn, and just one A/D convertor reads the sensors in turn.
If you go analogue sensors then each one needs to be calibrated on its actual cable (as you have to allow for cable resistance on anything over 1m). Also the A/D readings will need to be converted to temperatures, so you'll need lookup tables if you want actual temps rather than just alarm condition reached.