'type of algae has strong garlic like odour'
Members of the genus Chara are plant-like green algae found in lakes, rivers, and ponds all over the world. They have fun names like muskgrass or skunkweed on account of their intense unpleasant odour, which has been described as vaguely resembling that of garlic. Many sulfur-containing organic molecules (take for example, diallyl disulfide) tend to smell like garlic. Chara globularis is known to produce at least two organosulfur compounds, 4-methylthio-l,2-dithiolane and 5-methylthio-l,2,3-trithiane. Both can inhibit photosynthesis, so I'm guessing they're produced by the algae to help it compete with other sun-using aquatic organisms.
'are there pesticides smell like fish?'
Yes, at least one pesticide does indeed smell like fish. 2,4-D is herbicide discovered by the Allies during WII (fruit of their efforts to develop new chemical weapons) and used by the US during the Vietnam War (as an ingredient in Agent Orange). What makes 2,4-D so useful is it's way less toxic to grasses compared to other plants, meaning it can be applied to lawns and cereal crops in order to selectively disrupt the growth of their competitors (AKA weeds). While it's good at what it does, 2,4-D does not dissolve well in water. To fix this, chemists have played around with it a bit over the years. One of the more successful modifications was a salt, 2,4-D dimethylamine. It's very good at dissolving in water, but has an intense fishy odour because it contains a small amine. Amines are derivatives of ammonia found in, among other things, raw fish.
'why do some toxic chemicals smell sweet?'
Toxic chemicals with a sweet odour include benzene, carbon tetrachloride, chloropicrin, cyclosarin, and diborane. From a structural standpoint, they don't appear to have much in common:
- Benzene is an electron-rich ring of six carbon atoms
- Cyclosarin consists of a phosphorus atom attached to atoms of carbon, oxygen, and fluorine, as well as a ring of carbon atoms.
- Carbon tetrachloride and chloropicrin have no rings but contain a bunch of chlorine atoms.
- Diborane consists only of boron and hydrogen.
Sell CS. 2006. On the unpredictability of odor. Angewandte Chemie International Edition 45(38):6254-6261. [Full text]
Stonard RJ, Miller-Wideman MA. 1994. Herbicides and plant growth regulators. In: Agrochemicals from Natural Products, Ed. CRA Godfrey. CRC Press. [Link]
Volgas GC, Mack RE, Roberts JR. 2005. Benefits of a 2,4-D acid herbicide formulation. In: Pesticide Formulations and Delivery Systems, Ed. GR Goss. ASTM International. [Link]