300 liters is @ 80 gallons, and would only work temporarily for Vieja as a grow out tank. Although I'm not sure what species rainbow Vieja is, but all Vieja I've kept need more that 100 gallons as adults (preferably much more) especially if kept with other species of cichlids. Even the most mild mannered Vieja get aggressive when kept in too small a tank.
Some Geophagines might work, depending on your tap water chemistry.
Geophagus crassiblabrus, steinachneri, and pelligrini are found in the harder waters west of the Andes, which is similar to the water nicaraguensee inhabit.
If by rainbow Vieja you mean V breidhori, melanura, bifasciata, or others of that size, a minimum 600 liter tank would be more apropos as adults.
bifasciata
breidhori
young melanura