Wow I don't know of many plants that will survive in those conditions. The only ones I can think of are java ferns and maybe some anubias. These can also survive in brackish water tanks so they may be your answer.
On a side note do you have a drop checker to make sure what CO2 level you are maintaining in the tank. You will probably be ok if you use the above plants but it is dificult to keep a good amount of CO2 in a 55g with just DIY. ALso what are you using to dissolve the CO2. If it is just in a ladder type of device then chances are you are not near a CO2 level that will help plants.
On a side note do you have a drop checker to make sure what CO2 level you are maintaining in the tank. You will probably be ok if you use the above plants but it is dificult to keep a good amount of CO2 in a 55g with just DIY. ALso what are you using to dissolve the CO2. If it is just in a ladder type of device then chances are you are not near a CO2 level that will help plants.