Am guessing thats a 1feet cube tank.
Started off with this size too Stability is key.
1) There are hang on mechanical filters n skimmers that you can utilise. May wish to water change(10-20%) more often if the latter is not used.
2) 1 feet T5 lamps should suffice, those with higher Kelvin or some blueish hue.
3) Are you using test kits? Do get the important ones to find out the root of the problem. Temperature too.
4) Acclimatisation methods. As the tank is smaller, parameters do shift easily and way out of the normal seawater expectations. Acclimatise them longer and add livestocks only after parameters are stable. 2 lives in 2 days dont seem very coincidental n they are pretty hardy guys :<
Hope the above helps n all the best in this hobby.