Can you post a pic...better to find out the cause before adding more livestock IMO.
What is the stocking, what type of filter and filter media care, water change history-how much and how often, type of additive used. Any live plants, if so how many and what species/type. How long has the tank been setup/running. What kind of lights and photoperiod. If florescent light-how old are the light bulbs, kelvin and watts. Hard scape items-any real driftwood.
Old florescent bulbs can cause algae problems as well as too short a photoperiod.
Right now what I would recommend-water changes, depending on how long the tank has been neglected before you took over and the livestock-If neglected for a while-you can have normal chemistry changes that when changed too fast can kill the livestock. Most common issue will be with nitrate and pH-if you can check these two and compare to the source water can help with how much water can be changed in the beginning so not to shock and kill the livestock.