He said "wherever you looked it was just sheer carnage" and there had been "dead fish everywhere".
Severn Trent, which owns the pool, said it was working with a "third party", believed to be the source of the pollution.
The Environment Agency said it had been pumping oxygen into the pool since the pollution was discovered last week and levels of dissolved oxygen were improving.
But Mr Tomkinson said he believed that would need to continue "for a long, long time to come".
He said the pool had suffered from pollution coming in from drains for the last 10 years and there were a lot of questions to be asked.
His group has had boats out collecting the dead fish and he said they were finding them washed up on every side of the water.
"Being a fisherman myself it hits you", he said.
Mr Tomkinson said it would take "years and years" to return fish stock levels back to what they were.
He also feared the pollution would remain in the water.
"It'll sink to the bottom and it'll be in the silt now," he said.
Severn Trent said the pollution was not a result of its operations and it added: "We believe the source of the pollution is coming from a third party which we are now working with."
It said it had been working with the Environment Agency and other partners.