We applied seaweed to controlling the ammonia-nitrogen content in intensive shrimp aquaculture ponds in developing countries. Sterile Ulva sp. in Kanazawa Bay, Yokohama, Japan, was selected as model algae. The water content in the alga cells was almost the same as the previous result for Ulva lactuca. The rates of ammonia-nitrogen uptake by algae were measured under various conditions including tropical conditions, i.e., higher light flux density and temperature, to know the effects of these conditions. Sterile Ulva sp. could remove ammonia-nitrogen in culture medium effectively. The algae took up ammonia-nitrogen even under no light. Light enhanced assimilation of ammonia-nitrogen in the alga cell. The tropical condition enhanced ammonia-nitrogen uptake by the alga. The ammonia-nitrogen uptake rate decreased with an increasing ammonia-nitrogen content in the alga cell. Based on these experimental results, we studied the mechanism of ammonia-nitrogen uptake by algae in terms of a Michaelis-Menten model with an inhibitory effect. The ammonia-nitrogen uptake obeyed the Michaelis-Menten model with uncompetitive inhibition by the inhibitor working on the complex of ammonia-nitrogen and its carrier, and the ammonia-nitrogen content in an alga cell was positively related to the inhibitory effect on ammonia-nitrogen uptake. Consequently, the uptake by alga was proposed for controlling the ammonia-nitrogen content in shrimp aquaculture ponds in developing countries, and useful information for shrimp pond design was provided.